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[bot]]

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 :warning:

Additional details and impacted files

[](https://app.codecov.io/gh/CrayLabs/SmartSim/pull/607?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CrayLabs) ```diff @@ Coverage Diff @@ ## mli-feature #607 +/- ## ================================================ - Coverage 71.53% 53.35% -18.18% ================================================ Files 78 79 +1 Lines 6000 6187 +187 ================================================ - Hits 4292 3301 -991 - Misses 1708 2886 +1178 ``` | [Files with missing lines](https://app.codecov.io/gh/CrayLabs/SmartSim/pull/607?dropdown=coverage&src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CrayLabs) | Coverage Δ | | |---|---|---| | [smartsim/\_core/mli/message\_handler.py](https://app.codecov.io/gh/CrayLabs/SmartSim/pull/607?src=pr&el=tree&filepath=smartsim%2F_core%2Fmli%2Fmessage_handler.py&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CrayLabs#diff-c21hcnRzaW0vX2NvcmUvbWxpL21lc3NhZ2VfaGFuZGxlci5weQ==) | `0.00% <0.00%> (ø)` | | ... and [46 files with indirect coverage changes](https://app.codecov.io/gh/CrayLabs/SmartSim/pull/607/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=CrayLabs)