Add message description function from 1.x

[mulmarta]

Knowing content type of a message is very useful.

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 92.85714% with 4 lines in your changes missing coverage. Please review.

Project coverage is 90.26%. Comparing base (9c36934) to head (bbbb30e).

Files with missing lines	Patch %	Lines
mls-rs/src/group/framing.rs	92.85%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #255   +/-   ##
=======================================
  Coverage   90.26%   90.26%           
=======================================
  Files         181      181           
  Lines       32596    32652   +56     
=======================================
+ Hits        29422    29474   +52     
- Misses       3174     3178    +4     

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