Allow dynamic configuration of vschema acl

[dbussink]

This allows configuring the users for the vschema dynamically at runtime so you don't need to restart vtgate if this changes.

Related Issue(s)

Fixes #17350

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

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes missing coverage. Please review.

Project coverage is 67.48%. Comparing base (f746c48) to head (935e5f6).
Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vschemaacl/vschemaacl.go	90.90%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17333   +/-   ##
=======================================
  Coverage   67.48%   67.48%           
=======================================
  Files        1577     1577           
  Lines      253420   253458   +38     
=======================================
+ Hits       171022   171053   +31     
- Misses      82398    82405    +7     

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[GuptaManan100]

LGTM