Eliminate proposal "filtering" in favor of throwing hard errors

[tomleavy]

Description of changes:

We previously had code to filter received proposals when you commit so that the commit would work based on the maximum valid set. In practice is problematic because although the proposals that aren't consumed wind up in unused_proposals, it is very easy to ignore the fact that some expected change to the group was not enacted.

As an alternative approach we are moving towards hard failing if any proposal is not valid in the context of all the cached + by value proposals we have within a commit. This also allows us to remove the forked "lite" version of filtering + batch_edit on the tree.

Call-outs:

With this PR, there is no way to recover when you get into a bad state due to receiving a proposal that is not valid in the context of the other received proposals. This is not the final form of this feature change. Follow ups are as follows:

• Refactor the Error that is returned when a proposal is not valid to include a proposal ref and a reason. The reason should be an enum the represents the states described by the various MlsError cases we have currently.
• Add a function to the commit builder that allows you to remove a specific proposal by passing in a proposal ref. We currently have proposals_mut, but that isn't really the most convenient way to use that functionality.
• Add a function that mimics the older functionality by attempting to commit + remove any errored out proposal until the commit succeeds.

Testing:

Adjusted 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 98.70130% with 3 lines in your changes missing coverage. Please review.

Project coverage is 89.66%. Comparing base (c198d38) to head (581efd1).

Files with missing lines	Patch %	Lines
mls-rs/src/group/proposal_cache.rs	97.36%	1 Missing ⚠️
mls-rs/src/group/proposal_filter/filtering.rs	99.24%	1 Missing ⚠️
mls-rs/src/tree_kem/mod.rs	96.77%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           1.x-main     #245      +/-   ##
============================================
- Coverage     90.23%   89.66%   -0.57%     
============================================
  Files           176      176              
  Lines         31222    29737    -1485     
============================================
- Hits          28172    26664    -1508     
- Misses         3050     3073      +23     

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[tomleavy]

@mulmarta ignored two tests you can take a look at if you have a chance before I get back online