Group Concat function support for separator

[GuptaManan100]

Description

As pointed out in #16238 the query there panics the vtgate which eventually crashes it.
This PR does 2 fixes -

1. Adds support for separator field in group concat evaluation on the vtgate.
2. Fixes the error message the user receives on planning failure and prevents crashes.

Related Issue(s)

• Fixes Bug Report: Group_concat with separator crashes vtgate during planning #16238

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

Deployment Notes

[vitess-bot]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.59%. Comparing base (f118ba2) to head (e603162).
Report is 204 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16237      +/-   ##
==========================================
+ Coverage   68.40%   68.59%   +0.18%     
==========================================
  Files        1556     1544      -12     
  Lines      195121   197984    +2863     
==========================================
+ Hits       133479   135811    +2332     
- Misses      61642    62173     +531     

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[harshit-gangal]

I understand that we want to add vitess-tester tests, but there is a cost of adding new test files. For a single query testing should be not update the existing group_concat test in TestGroupConcatAggregation

[GuptaManan100]

I think we should use vitess_tester for all new tests, because of how much easier it is. Currently, the workflow is running in 6m 24s seconds, even after adding this test.

[GuptaManan100]

I'd rather like to move all the older tests to vitess_tester