make the results observer easier to extend, and give it a life cycle

[systay]

Description

To make the resultsObserver interface really useful, we need to get some additional information.
This PR adds the query that started the work on vtgate as a start parameter to the creation of the resultObserver, and adds a Close() method to the interface so that the observer gets notified once no more results are incoming.

Related Issue(s)

Following #16638

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

Hello reviewers! 👋 Please follow this checklist when reviewing this Pull Request.

General

• Ensure that the Pull Request has a descriptive title.
• Ensure there is a link to an issue (except for internal cleanup and flaky test fixes), new features should have an RFC that documents use cases and test cases.

Tests

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Documentation

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• There should be some code comments as to why things are implemented the way they are.
• There should be a comment at the top of each new or modified test to explain what the test does.

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• vtctl command output order should be stable and awk-able.

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.93%. Comparing base (a513601) to head (be19f48).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vcursor_impl.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16736      +/-   ##
==========================================
- Coverage   68.93%   68.93%   -0.01%     
==========================================
  Files        1565     1565              
  Lines      201745   201749       +4     
==========================================
- Hits       139075   139073       -2     
- Misses      62670    62676       +6     

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[systay]

We'll solve this another way.