Ability to add group context extensions

[neekolas]

Problem

OpenMLS doesn't allow group creators to manually set group_context_extensions at the time of group creation. There are special functions for setting the ratchet tree extension and the required capabilities extension, but no generic interface for custom group_context_extensions like ProtectedMetadata that can be configured at the application level.

Solution

• Adds the ability to add group_context_extensions on the MlsGroupConfigBuilder
• Pass those extensions down to the CoreGroup builder
• Adds a test to verify the extensions are being correctly set

[codecov-commenter]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (d327edf) 86.21% compared to head (b0e3742) 86.26%.

Files	Patch %	Lines
openmls/src/group/mls_group/config.rs	62.50%	3 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #11      +/-   ##
==========================================
+ Coverage   86.21%   86.26%   +0.04%     
==========================================
  Files         169      169              
  Lines       19024    19074      +50     
==========================================
+ Hits        16402    16454      +52     
+ Misses       2622     2620       -2     

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