chore: Move protobuf files to separate crate

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Splitting the project into smaller crates has some benefits:

• Helps with build times
• Allows other Rust projects to re-use parts of Comet
• Improves maintainability and allows for debugging/testing modules in isolation

What changes are included in this PR?

• Move protobuf files and build.rs into separate crate
• No functional changes

How are these changes tested?

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 33.65%. Comparing base (b6d868c) to head (04735f6).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #661   +/-   ##
=========================================
  Coverage     33.65%   33.65%           
  Complexity      842      842           
=========================================
  Files           109      109           
  Lines         42573    42573           
  Branches       9334     9334           
=========================================
  Hits          14328    14328           
  Misses        25297    25297           
  Partials       2948     2948           

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