Add functions to retrieve avatars

[Schmiddiii]

A few TODOs are left which I am unsure about:

• When fetching the profile avatar, I am unsure about using the identified or unidentified push service. Note: Unidentified seems to work. I thus chose this.
• Is there some way of knowing when a profile or profile avatar (or group avatar) gets outdated, to remove that from the cache.
• This may require a migration to remove the already cached profiles, otherwise no avatars for profiles will ever be fetched.
• Is there maybe some better format to use for avatars instead of Vec<u8>, e.g. Bytes?

[gferon]

Thanks a lot! We could definitely introduce a convenience wrapper type for avatars if you think it's worth it.

[Schmiddiii]

I have now updated this PR. A few notes:

• Using unidentified instead of identified seems to work for profile fetching, I therefore used unidentified (still don't know what is correct though).
• Added the migration mentioned.
• Added type AvatarBytes = Vec<u8>.
• Prevented a panic in libsignal-service-rs when fetching a group avatar with empty path (see also Fix panic for fetching group avatar libsignal-service-rs#291).

I am still unsure if there exists some message that notifies when a profile/avatar gets out-of-date.

[Schmiddiii]

Note from WF chat: Profile updates seem to be done by WF and SA on a schedule. While presage does not yet have such a schedule, we could certainly do something similar. But I think this can also be done in another MR, I would therefore call this mergeable (if you don't have any comments on it).

See also:

• https://gitlab.com/whisperfish/whisperfish/-/blob/main/whisperfish/src/worker/profile_refresh.rs?ref_type=heads
• https://github.com/signalapp/Signal-Android/blob/09b9349f6c0cf02688a79d8c2c9edeb8b32dd3cf/app/src/main/java/org/thoughtcrime/securesms/database/RecipientDatabase.kt#L3209

[gferon]

Thanks a lot for your work! I fixed some cosmetic things and some clippy warnings, and will merge it now.