Add support for global routing in describe statement

[harshit-gangal]

Description

This PR adds support for global routing in describe statement

Example:
describe tbl_name

If tbl_name is unique it should be routed to the correct keyspace.

Related Issue(s)

• Fixes Describe Statement: Global routing support #17514

Checklist

• 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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.69%. Comparing base (db4fca3) to head (eff08a4).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17510   +/-   ##
=======================================
  Coverage   67.68%   67.69%           
=======================================
  Files        1584     1584           
  Lines      254630   254626    -4     
=======================================
+ Hits       172356   172374   +18     
+ Misses      82274    82252   -22     

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[frouioui]

We need to modify TestDescribeVindex in vtgate/misc_test.go to assert the new error type we are returning. Otherwise LGTM!