Add Keys sync message

[direc85]

Take two, intentionally (for development convenience) branched of an old commit - the one Whisperfish main currently uses.

I tried to remove passing self-recipient, but couldn't figure out a way to get it in the method. I think that would require setting self service address to sender, which would break API, so I'm not delving into that currently.

Real-world tests performed:

• Linking Signal Desktop (tested with Whisperfish -> SD)
• Responding to Signal Desktop Keys sync request
• Sending a Keys sync request

[direc85]

Added WIP for MasterKey and StorageServiceKey, as well as MasterKeyStorage, which is probably done in some funky manner. Improvements and corrections welcome...

[gferon]

Thanks! At the risk of missing the point, why was this suddenly needed? 😄

[direc85]

These are needed to properly support the Keys sync message, which is needed for linking nowadays... Ruben suggested making these into a trait, I'm not at all sure I did that in a good way.

Basically this part should be quite ready, but there's still the "where are these actually used" side of things...

[direc85]

We still must test this code, but review-wise this PR can be considered ready.

[codecov]

Codecov Report

Attention: Patch coverage is 51.21951% with 20 lines in your changes missing coverage. Please review.

Project coverage is 23.72%. Comparing base (3b8656a) to head (b8d7761).
Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
libsignal-service/src/master_key.rs	51.21%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #310      +/-   ##
==========================================
+ Coverage   16.77%   23.72%   +6.94%     
==========================================
  Files          36       32       -4     
  Lines        3111     1888    -1223     
==========================================
- Hits          522      448      -74     
+ Misses       2589     1440    -1149     

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[rubdos]

Some generalizations, otherwise looking clean to me! :-)