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.

[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 :warning:
mls-rs/src/group/mod.rs	94.20%	4 Missing :warning:
mls-rs/src/group/message_processor.rs	86.36%	3 Missing :warning:

Additional details and impacted files

```diff @@ 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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