VMDistributed: support RequestsLoadBalancer

[vrutkovs]

Update the VMAuth to use static path to requests LB when VMCluster has RequestsLoadBalancer enabled.

Fixes #1814

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Summary by cubic

VMAuth now routes /select and tenant admin requests to the VMCluster Requests LoadBalancer via a static service URL when enabled. Fixes #1814.

• Bug Fixes
  • Use StaticRef to http://vmclusterlb-..svc: for /select and /admin/tenants when RequestsLoadBalancer is enabled (default port 8427; custom port supported).
  • Fall back to VMCluster/vmselect CRD target when the LB is disabled or select balancing is turned off.
  • Sort TargetRefs consistently for both Static and CRD backends; added tests for LB on/off, custom port, exclusions, and sorting.

^{Written for commit a9ebb0e. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[vrutkovs]

Lets hold this one for now, as ideally VMCluster/select would detect this automatically