feat: Deduplicate blob files in the JsonRPC API

[Hocuri]

This makes it so that files will be deduplicated when using the JsonRPC API.

@nicodh and @WofWca you know the Desktop code and how it is using the API, so, you can probably tell me whether this is a good way of changing the JsonRPC code - feel free to push changes directly to this PR here!

This PR here changes the existing functions instead of creating new ones; we can alternatively create new ones if it allows for a smoother transition.

This brings a few changes:

• If you pass a file that is already in the blobdir, it will be renamed to <hash>.<extension> immediately (previously, the filename on the disk stayed the same)
• If you pass a file that's not in the blobdir yet, it will be copied to the blobdir immediately (previously, it was copied to the blobdir later, when sending)
  • If you create a file and then pass it to create_message(), it's better to directly create it in the blobdir, since it doesn't need to be copied.
• You must not write to the files after they were passed to core, because otherwise, the hash will be wrong. So, if Desktop recodes videos or so, then the video file mustn't just be overwritten. What you can do instead is write the recoded video to a file with a random name in the blobdir and then create a new message with the new attachment. If needed, we can also create a JsonRPC for set_file_and_deduplicate() that replaces the file on an existing message.

In order to test whether everything still works, the desktop issue has a list of things to test: deltachat/deltachat-desktop#4498
Core issue: #6265

[Hocuri]

Not sure whether it's necessary to change this function and misc_set_draft() since it's going to be replaced anyway?

Is this function and misc_set_draft() still in use in the Desktop UI?

[link2xt]

Currently it is still used, did a quick grep for miscSetDraft.