List join predicates used in GetVExplainKeys

[frouioui]

Description

This PR enhances GetVExplainKeys to also list the join predicates that were used in the given query. This enable us to achieve vitessio/vt#43 in vt keys and vt benchstat. Example output and usage were detailed in vitessio/vt#43.

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 71.15385% with 15 lines in your changes missing coverage. Please review.

Project coverage is 67.52%. Comparing base (1a4f2b9) to head (cb8ee86).
Report is 12 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/planbuilder/operators/keys.go	71.15%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #17130      +/-   ##
==========================================
+ Coverage   67.44%   67.52%   +0.07%     
==========================================
  Files        1571     1571              
  Lines      252197   252618     +421     
==========================================
+ Hits       170103   170581     +478     
+ Misses      82094    82037      -57     

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