vtorc: add index for inst.ReadInstanceClusterAttributes table scan

[timvaillancourt]

Description

This PR adds an index for (hostname, port) on the VTOrc database_instance table to avoid an expensive full table scan caused by inst.ReadInstanceClusterAttributes looking for rows by hostname and port

In the future I'd like to do something smarter as discussed in #17843, but for now this index performs much better than not-having it

Before w/2000~ tablets (18% of profile time)

After w/2000~ tablets (2.6% of profile time)

Related Issue(s)

#17330

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
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• Tests were added or are not required
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• Documentation was added or is not required

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.47%. Comparing base (81ce29c) to head (e751f29).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17866   +/-   ##
=======================================
  Coverage   67.46%   67.47%           
=======================================
  Files        1593     1594    +1     
  Lines      258885   258978   +93     
=======================================
+ Hits       174652   174734   +82     
- Misses      84233    84244   +11     

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