Add API for examining roster of ExportedTree

[mulmarta]

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 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 90.18%. Comparing base (539cc5c) to head (b1b2e7b).

Files with missing lines	Patch %	Lines
mls-rs/src/group/exported_tree.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   90.19%   90.18%   -0.02%     
==========================================
  Files         180      180              
  Lines       32301    32308       +7     
==========================================
+ Hits        29134    29136       +2     
- Misses       3167     3172       +5     

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