feat(core): implement parallel variants of attribute storage traits & structs

[imrn99]

• add ParUnknownAttributeStorage, with update method sigs
• add ParAttributeStorage, with update method sigs
• implement these for a new struct, PAttrSparseVec

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.45238% with 11 lines in your changes missing coverage. Please review.

Project coverage is 82.03%. Comparing base (ab03601) to head (65c688c).

Files with missing lines	Patch %	Lines
honeycomb-core/src/attributes/collections.rs	90.27%	7 Missing ⚠️
honeycomb-core/src/attributes/traits.rs	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #199      +/-   ##
==========================================
+ Coverage   81.67%   82.03%   +0.35%     
==========================================
  Files          38       38              
  Lines        5342     5510     +168     
==========================================
+ Hits         4363     4520     +157     
- Misses        979      990      +11     

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[cedricchevalier19]

Can you test the usefulness of the lock?

Can you comment on why you chose this Vec<RwLock<>> solution.
A RwLock<Vec<>> solution is also possible, can you comment on the differences?

[imrn99]

Can you test the usefulness of the lock?

I'm not sure if I can test it easily, but I can demonstrate its usefulness

Can you comment on why you chose this Vec<RwLock<>> solution. A RwLock<Vec<>> solution is also possible, can you comment on the differences?

Given the conversation in #199, I'll close this PR and use the same approach here (change the original struct). I'll do everything in #201, with a dedicated comment per change in the PR