refactor: Simplify RPC handling in SnapController

[FrederikBolding]

Stop storing an RPC handler in the runtime for Snaps, instead just move the handler code directly to handleRequest. Also removes the RequestQueue previously used during the Snap starting up, instead we let as many requests as needed wait for startPromise. This simplifies RPC request routing significantly.

Recommend reviewing this with whitespace changes not displayed 😄

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.13%. Comparing base (4acb5c3) to head (dff1689).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3397      +/-   ##
==========================================
+ Coverage   98.12%   98.13%   +0.01%     
==========================================
  Files         399      398       -1     
  Lines       10961    10924      -37     
  Branches     1725     1716       -9     
==========================================
- Hits        10755    10720      -35     
+ Misses        206      204       -2     

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