Initial MLI schemas and MessageHandler class

[AlyssaCote]

This PR adds the Capnproto schemas and initial MessageHandler class and tests. This is very much a work in progress!

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 204 lines in your changes missing coverage. Please review.

Project coverage is 53.35%. Comparing base (7d995bb) to head (60faf5d).
Report is 39 commits behind head on mli-feature.

Files with missing lines	Patch %	Lines
smartsim/_core/mli/message_handler.py	0.00%	204 Missing ⚠️

Additional details and impacted files

@@               Coverage Diff                @@
##           mli-feature     #607       +/-   ##
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- Coverage        71.53%   53.35%   -18.18%     
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  Files               78       79        +1     
  Lines             6000     6187      +187     
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- Hits              4292     3301      -991     
- Misses            1708     2886     +1178     

Files with missing lines	Coverage Δ
smartsim/_core/mli/message_handler.py	0.00% <0.00%> (ø)

... and 46 files with indirect coverage changes

[mellis13]

Just some quick questions/comments/suggestions

[al-rigazzi]

Looks good! Some general comments, possibly tending to over-engineering.

[al-rigazzi]

I'm slightly concerned by extensibility/maintainability of this type: do we need to keep coming to this portion of the code and add new XyzRequestAttributes every time we support a new library? Is there a more flexible way? ("No" is an acceptable answer).

[AlyssaCote]

I am also concerned about the extensibility/maintainability of this type! Currently I think we will just need to come back and add to it because I need to check the passed in schema name and set the union field depending on that. So it might be no for now, but hopefully I can come up with a better way in the future.