File deduplication (#6332)

When receiving messages, blobs will be deduplicated with the new
function `create_and_deduplicate_from_bytes()`. For sending files, this
adds a new function `set_file_and_deduplicate()` instead of
deduplicating by default.

This is for
https://github.com/deltachat/deltachat-core-rust/issues/6265; read the
issue description there for more details.

TODO:
- [x] Set files as read-only
- [x] Don't do a write when the file is already identical
- [x] The first 32 chars or so of the 64-character hash are enough. I
calculated that if 10b people (i.e. all of humanity) use DC, and each of
them has 200k distinct blob files (I have 4k in my day-to-day account),
and we used 20 chars, then the expected value for the number of name
collisions would be ~0.0002 (and the probability that there is a least
one name collision is lower than that) [^1]. I added 12 more characters
to be on the super safe side, but this wouldn't be necessary and I could
also make it 20 instead of 32.
- Not 100% sure whether that's necessary at all - it would mainly be
necessary if we might hit a length limit on some file systems (the
blobdir is usually sth like
`accounts/2ff9fc096d2f46b6832b24a1ed99c0d6/dc.db-blobs` (53 chars), plus
64 chars for the filename would be 117).
- [x] "touch" the files to prevent them from being deleted
- [x] TODOs in the code

For later PRs:
- Replace `BlobObject::create(…)` with
`BlobObject::create_and_deduplicate(…)` in order to deduplicate
everytime core creates a file
- Modify JsonRPC to deduplicate blob files
- Possibly rename BlobObject.name to BlobObject.file in order to prevent
confusion (because `name` usually means "user-visible-name", not "name
of the file on disk").

[^1]: Calculated with both https://printfn.github.io/fend/ and
https://www.geogebra.org/calculator, both of which came to the same
result
([1](https://github.com/user-attachments/assets/bbb62550-3781-48b5-88b1-ba0e29c28c0d),

[2](https://github.com/user-attachments/assets/82171212-b797-4117-a39f-0e132eac7252))

---------

Co-authored-by: l <link2xt@testrun.org>

deltachat-ffi/deltachat.h

@@ -4756,6 +4756,31 @@ void            dc_msg_set_override_sender_name(dc_msg_t* msg, const char* name)
 void            dc_msg_set_file               (dc_msg_t* msg, const char* file, const char* filemime);
 
 
+/**
+ * Sets the file associated with a message.
+ *
+ * If `name` is non-null, it is used as the file name
+ * and the actual current name of the file is ignored.
+ *
+ * If the source file is already in the blobdir, it will be renamed,
+ * otherwise it will be copied to the blobdir first.
+ *
+ * In order to deduplicate files that contain the same data,
+ * the file will be named as a hash of the file data.
+ *
+ * NOTE:
+ * - This function will rename the file. To get the new file path, call `get_file()`.
+ * - The file must not be modified after this function was called.
+ *
+ * @memberof dc_msg_t
+ * @param msg The message object. Must not be NULL.
+ * @param file The path of the file to attach. Must not be NULL.
+ * @param name The original filename of the attachment. If NULL, the current name of `file` will be used instead.
+ * @param filemime The MIME type of the file. NULL if you don't know or don't care.
+ */
+void            dc_msg_set_file_and_deduplicate(dc_msg_t* msg, const char* file, const char* name, const char* filemime);
+
+
 /**
  * Set the dimensions associated with message object.
  * Typically this is the width and the height of an image or video associated using dc_msg_set_file().