feat: receive pre-messages and adapt download on demand

fix python lint errors

receive pre-mesages, start with changes to imap loop.

refactor: move download code from `scheduler.rs` to `download.rs`, also
move `get_msg_id_by_rfc724_mid` to `MsgId::get_by_rfc724_mid`

`MAX_FETCH_MSG_SIZE` is no longer unused

Parse if it is a pre-message or full-message

start with receiving logic

get rid of `MsgId::get_by_rfc724_mid` because it was a duplicate of
`message::rfc724_mid_exists`

docs: add hint to `MimeMessage::from_bytes` stating that it has
side-effects.

receiving full message

send and receive `attachment_size` and set viewtype to text in
pre_message

metadata as struct in pre-message in header. And fill params that we can
already fill from the metadata. Also add a new api to check what
viewtype the message will have once downloaded.

api: jsonrpc: add `full_message_view_type` to `Message` and
`MessageInfo`

make PreMsgMetadata.to_header_value not consume self/PreMsgMetadata

add api to merge params

on download full message: merge new params into old params and remove
full-message metadata params

move tests to `src/tests/pre_messages.rs`

dynamically allocate test attachment bytes

fix detection of pre-messages. (it looked for the ChatFullMessageId
header in the unencrypted headers before)

fix setting dl state to avaiable on pre-messages

fix: save pre message with rfc724_mid of full message als disable
replacement for full messages

add some receiving tests and update test todo for premessage metadata

test: process full message before pre-message

test receive normal message

some serialization tests for PreMsgMetadata

remove outdated todo comment

test that pre-message contains message text

PreMsgMetadata: test_build_from_file_msg and test_build_from_file_msg

test: test_receive_pre_message_image

Test receiving the full message after receiving an edit after receiving
the pre-message

test_reaction_on_pre_message

test_full_download_after_trashed

test_webxdc_update_for_not_downloaded_instance

simplify fake webxdc generation in
test_webxdc_update_for_not_downloaded_instance

test_markseen_pre_msg

test_pre_msg_can_start_chat and test_full_msg_can_start_chat

test_download_later_keeps_message_order

test_chatlist_event_on_full_msg_download

fix download not working

log splitting into pre-message

add pre-message info to text when loading from db. this can be disabled
with config key `hide_pre_message_metadata_text` if ui wants to display
it in a prettier way.

update `download_limit` documentation

more logging: log size of pre and post messages

rename full message to Post-Message

split up the pre-message tests into multiple files

dedup test code by extracting code to create test messages into util
methods

remove post_message_view_type from api, now it is only used internally
for tests

remove `hide_pre_message_metadata_text` config option, as there
currently is no way to get the full message viewtype anymore

Update src/download.rs
resolve comment

use `parse_message_id` instead of removing `<>`parenthesis it manually

fix available_post_msgs gets no entries
handle forwarding and add a test for it.

convert comment to log warning event on unexpected download failure

add doc comment to `simple_imap_loop`

more logging

handle saving pre-message to self messages and test.

deltachat-ffi/deltachat.h

@@ -488,11 +488,11 @@ char*           dc_get_blobdir               (const dc_context_t* context);
  *                    0=use IMAP IDLE if the server supports it.
  *                    This is a developer option used for testing polling used as an IDLE fallback.
  * - `download_limit` = Messages up to this number of bytes are downloaded automatically.
- *                    For larger messages, only the header is downloaded and a placeholder is shown.
+ *                    For messages with large attachements, two messages are sent:
+ *                    a Pre-Message containing metadata and a Post-Message containing the attachment.
+ *                    Pre-Messages are always downloaded and show a placeholder message 
  *                    These messages can be downloaded fully using dc_download_full_msg() later.
- *                    The limit is compared against raw message sizes, including headers.
- *                    The actually used limit may be corrected
- *                    to not mess up with non-delivery-reports or read-receipts.
+ *                    Post-Messages are automatically downloaded if they are smaller than the download_limit.
  *                    0=no limit (default).
  *                    Changes affect future messages only.
  * - `protect_autocrypt` = Enable Header Protection for Autocrypt header.
@@ -4310,7 +4310,8 @@ char*             dc_msg_get_webxdc_info      (const dc_msg_t* msg);
 
 /**
  * Get the size of the file. Returns the size of the file associated with a
- * message, if applicable.
+ * message, if applicable. 
+ * If message is a pre-message, then this returns size of the to be downloaded file.
  *
  * Typically, this is used to show the size of document files, e.g. a PDF.
  *