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>

Cargo.lock

@@ -571,9 +571,9 @@ dependencies = [
 
 [[package]]
 name = "blake3"
-version = "1.5.4"
+version = "1.5.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "d82033247fd8e890df8f740e407ad4d038debb9eb1f40533fffb32e7d17dc6f7"
+checksum = "b8ee0c1824c4dea5b5f81736aff91bae041d2c07ee1192bec91054e10e3e601e"
 dependencies = [
  "arrayref",
  "arrayvec",
@@ -984,9 +984,9 @@ dependencies = [
 
 [[package]]
 name = "constant_time_eq"
-version = "0.3.0"
+version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "f7144d30dcf0fafbce74250a3963025d8d52177934239851c917d29f1df280c2"
+checksum = "7c74b8349d32d297c9134b8c88677813a227df8f779daa29bfc29c183fe3dca6"
 
 [[package]]
 name = "convert_case"
@@ -1316,6 +1316,7 @@ dependencies = [
  "async-smtp",
  "async_zip",
  "base64 0.22.1",
+ "blake3",
  "brotli",
  "bytes",
  "chrono",