Support receiving own proposals without an error

[mulmarta]

To make implementing DS's easier, this PR allows clients to receive their own proposals.

• For plaintext proposals, this is trivial, since they can be processed the same way as proposals from other clients
• For ciphertext proposals, we cache a list of hashes of all encrypted proposals we generated and just return Ok when processing one of them.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 and MIT license.

[stefunctional]

Thank you!

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 91.09589% with 13 lines in your changes missing coverage. Please review.

Project coverage is 89.65%. Comparing base (7c2926e) to head (5a24654).

Files	Patch %	Lines
mls-rs/src/group/message_hash.rs	57.14%	6 Missing ⚠️
mls-rs/src/group/mod.rs	94.20%	4 Missing ⚠️
mls-rs/src/group/message_processor.rs	86.36%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #154      +/-   ##
==========================================
+ Coverage   89.62%   89.65%   +0.02%     
==========================================
  Files         174      175       +1     
  Lines       31497    31578      +81     
==========================================
+ Hits        28230    28310      +80     
- Misses       3267     3268       +1     

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