Add support for view routing rules

[mhamza15]

Description

Adds support for view routing rules. View routing rules can be applied in the same way as tables:

{
  "rules": [
    {
      "from_table": "my_view",
      "to_tables": ["target_ks.my_view"]
    }
  ]
}

When routing rules are being built in VTGate, the from_table is first checked to see if it's a view. If it is, it's added into the local VSchema in a new ViewRoutingRules field:

type VSchema struct {
    // ...

    RoutingRules map[string]*RoutingRule `json:"routing_rules"`

    ViewRoutingRules map[string]*ViewRoutingRule `json:"view_routing_rules"`

    // ...
}

A view routing rule is defined as:

type ViewRoutingRule struct {
    TargetKeyspace string
    TargetViewName string
}

At query time, views are rewritten into the target view's definition rather than the source view. For example, for this view:

create view user_view as select id, name from user

And this routing rule:

{
  "rules": [
    {
      "from_table": "user_view",
      "to_tables": ["target_ks.user_view"]
    }
  ]
}

The query:

select * from user_view

Would be rewritten into:

select * from (select col1, col2 from target_ks.user_view) as user_view

See the RFC for more information.

Related Issue(s)

Closes #19097

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

AI Disclosure

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[codecov]

Codecov Report

❌ Patch coverage is 80.95238% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.88%. Comparing base (fa770f9) to head (ef0a54b).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
go/vt/vtgate/vindexes/vschema.go	83.33%	5 Missing ⚠️
go/test/vschemawrapper/vschema_wrapper.go	66.66%	1 Missing ⚠️
go/vt/sqlparser/normalizer.go	80.00%	1 Missing ⚠️
go/vt/vtgate/executorcontext/vcursor_impl.go	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #19104   +/-   ##
=======================================
  Coverage   69.87%   69.88%           
=======================================
  Files        1613     1613           
  Lines      216002   216048   +46     
=======================================
+ Hits       150939   150981   +42     
- Misses      65063    65067    +4     

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