fix: handle info_schema routing

[systay]

Description

In order to handle I_S tables/views correctly, we have to do thing different than we do for normal tables.
These queries can be sent to any shard on any keyspace.

Backports

This is a relatively low-risk change to backport, so I suggest we backport it at least to v18 and v19.

Related Issue(s)

Fixes #15900

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

Review Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 78.94737% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 68.46%. Comparing base (cbf89bd) to head (cf8039e).

Files	Patch %	Lines
go/vt/vtgate/planbuilder/vexplain.go	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15899      +/-   ##
==========================================
+ Coverage   68.43%   68.46%   +0.02%     
==========================================
  Files        1559     1559              
  Lines      196753   196763      +10     
==========================================
+ Hits       134657   134713      +56     
+ Misses      62096    62050      -46     

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