feat(relayer): add indexer in MySQL (#15927)

[xiaodino]

Cherry pick #15927

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (alpha-6@124e472). Click here to learn what that means.

Additional details and impacted files

@@            Coverage Diff             @@
##             alpha-6   #15990   +/-   ##
==========================================
  Coverage           ?   27.04%           
==========================================
  Files              ?      102           
  Lines              ?     6093           
  Branches           ?        0           
==========================================
  Hits               ?     1648           
  Misses             ?     4265           
  Partials           ?      180           

Flag	Coverage Δ		*Carryforward flag
eventindexer	17.28% <ø> (?)		Carriedforward from 739a268
guardian-prover-health-check	36.13% <ø> (?)		Carriedforward from 739a268
relayer	37.97% <ø> (?)

*This pull request uses carry forward flags. Click here to find out more.

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