Add a serde feature and derive serde for most structs used by providers

[mulmarta]

Allows more convenient providers implementations, e.g. with JSONs for debug. Changes mechanical.

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[codecov-commenter]

Codecov Report

Attention: 16 lines in your changes are missing coverage. Please review.

Comparison is base (105faf3) 91.47% compared to head (393006d) 91.46%.
Report is 2 commits behind head on main.

Files	Patch %	Lines
mls-rs/src/tree_kem/math.rs	90.24%	8 Missing ⚠️
mls-rs/src/group/commit.rs	81.81%	2 Missing ⚠️
mls-rs/src/group/mod.rs	91.66%	2 Missing ⚠️
mls-rs/src/group/secret_tree.rs	97.01%	2 Missing ⚠️
mls-rs/src/tree_kem/parent_hash.rs	93.75%	1 Missing ⚠️
mls-rs/src/tree_kem/tree_hash.rs	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #76      +/-   ##
==========================================
- Coverage   91.47%   91.46%   -0.01%     
==========================================
  Files         169      168       -1     
  Lines       30319    30307      -12     
==========================================
- Hits        27733    27720      -13     
- Misses       2586     2587       +1     

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[tomleavy]

I think we just need to put a base64 transform anywhere that we have serde where we use bytevec for the mls side