replace Any by UnknownAttributeStorage to handle collections in attribute manager

[imrn99]

• title
• implement new methods to handle manager-wide updates of attribute collections
• closes Figure out how to downcast each Box<dyn Any> object correctly when iterating over all storages #93

Note: It looks like downcast-rs isn't used, but if you remove the Downcast trait from UnknownAttributeStorage, the code won't compile.

Scope

• Code

Type of change

• New feature(s)
• Refactor

Other

• New dependency: downcast-rs; Necessary until trait_upcasting is stabilized Tracking issue for dyn upcasting coercion rust-lang/rust#65991

Necessary follow-up

• Needs testing
• Other: with these new methods, it should be possible to add the manager to CMap2 & wire everything up to handle attributes automatically

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 120 lines in your changes missing coverage. Please review.

Project coverage is 79.46%. Comparing base (ea6dc58) to head (b481b90).

Files	Patch %	Lines
honeycomb-core/src/attributes/manager.rs	0.00%	120 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #97      +/-   ##
==========================================
- Coverage   83.00%   79.46%   -3.55%     
==========================================
  Files          27       27              
  Lines        2601     2717     +116     
==========================================
  Hits         2159     2159              
- Misses        442      558     +116     

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