Remove PreSharedKeyStorage from Client and Group

[tomleavy]

Issues:

Work towards #211
Resolves #215

Description of changes:

Remove the PreSharedKeyStorage trait from Client and Group operations

Call-outs:

There are some obvious places where there can be optimizations like removing clones or passing an iterator instead of a Vec. Since there are other changes in those areas I am keeping this PR limited to just getting things working and we can take care of those other items later.

Testing:

Adapted all existing tests

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 97.21254% with 16 lines in your changes missing coverage. Please review.

Project coverage is 90.34%. Comparing base (7b16a02) to head (aac5b68).

Files with missing lines	Patch %	Lines
mls-rs/src/group/commit/processor.rs	86.27%	7 Missing ⚠️
mls-rs/src/group/external_commit.rs	96.38%	3 Missing ⚠️
mls-rs/src/group/mod.rs	97.56%	3 Missing ⚠️
mls-rs/src/group_joiner.rs	95.89%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           1.x-main     #238      +/-   ##
============================================
+ Coverage     90.21%   90.34%   +0.12%     
============================================
  Files           178      177       -1     
  Lines         31175    31318     +143     
============================================
+ Hits          28125    28294     +169     
+ Misses         3050     3024      -26     

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