[Bugfix] ExternalGroup was not able to issue local proposals

[mulmarta]

The line

let can_propose = match (proposer, by_ref) {
    ....
    (Sender::External(_), false) => false,

is wrong and prevents the server from issuing Local proposals

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 70.96774% with 9 lines in your changes missing coverage. Please review.

Project coverage is 90.12%. Comparing base (0532380) to head (71125a6).

Files with missing lines	Patch %	Lines
mls-rs/src/group/proposal_cache.rs	57.14%	6 Missing ⚠️
mls-rs/src/group/proposal_filter/filtering.rs	82.35%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
- Coverage   90.16%   90.12%   -0.04%     
==========================================
  Files         175      175              
  Lines       31448    31446       -2     
==========================================
- Hits        28354    28341      -13     
- Misses       3094     3105      +11     

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