Fix optional Event.type

[penso]

Fixes #1415

• Referenced an issue explaining the need for the change
• Updated all relevant documentation in docs
• Updated all code comments where relevant
• Wrote tests
• Added entry in .changelog/

[hdevalence]

I'm not sure this is an optimal solution, because it's recreating the data format by hand rather than deriving it from the source of truth (the proto messages). See #1415 (comment)

[penso]

Went for the shortest fix to an immediate issue. But anyone else, please feel free to fix it differently.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.22222% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 59.5%. Comparing base (99ed0b9) to head (90ddb0a).
Report is 10 commits behind head on main.

❗ Current head 90ddb0a differs from pull request most recent head 775d001. Consider uploading reports for the commit 775d001 to get more accurate results

Files	Patch %	Lines
rpc/tests/dydx_fixtures.rs	97.2%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #1416     +/-   ##
=======================================
- Coverage   60.1%   59.5%   -0.6%     
=======================================
  Files        271     272      +1     
  Lines      26221   26795    +574     
=======================================
+ Hits       15768   15963    +195     
- Misses     10453   10832    +379     

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