Add GroupInfo and Welcome message decryption APIs

[mulmarta]

Add APIs to decrypt group info from welcome message (without tree) and validate group info (without tree). They may be useful.

Code in group/mod.rs was copied to a new function but not modified

Testing:

Added unit test

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 95.33898% with 11 lines in your changes missing coverage. Please review.

Project coverage is 90.00%. Comparing base (e1fba48) to head (f483afd).

Files with missing lines	Patch %	Lines
mls-rs/src/group/mod.rs	92.79%	8 Missing ⚠️
mls-rs/src/group/util.rs	93.75%	2 Missing ⚠️
mls-rs/src/client.rs	98.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #188      +/-   ##
==========================================
+ Coverage   89.95%   90.00%   +0.04%     
==========================================
  Files         175      175              
  Lines       31106    31266     +160     
==========================================
+ Hits        27981    28140     +159     
- Misses       3125     3126       +1     

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