Don't take ownership for messages where not necessary

[mulmarta]

Removes a bunch of clone's. Receiving message still takes ownership due to wide-spread usage of AuthenticatedContent which takes ownership even though it wouldn't need to.

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 80.74534% with 31 lines in your changes are missing coverage. Please review.

Project coverage is 88.87%. Comparing base (13eba2c) to head (ac27866).

Files	Patch %	Lines
mls-rs/src/group/util.rs	63.46%	19 Missing ⚠️
mls-rs/src/group/message_processor.rs	12.50%	7 Missing ⚠️
mls-rs-uniffi/src/lib.rs	0.00%	2 Missing ⚠️
mls-rs/src/client.rs	90.00%	1 Missing ⚠️
mls-rs/src/group/framing.rs	80.00%	1 Missing ⚠️
mls-rs/src/group/mod.rs	97.72%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   88.88%   88.87%   -0.02%     
==========================================
  Files         171      171              
  Lines       31194    31109      -85     
==========================================
- Hits        27727    27648      -79     
+ Misses       3467     3461       -6     

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