[federation] Add multi-cluster routing support for federated broker queries

[shauryachats]

This PR is a part of the Cross Cluster Federation in Pinot as proposed here: https://github.com/apache/pinot/issues/16456

Overview

Introduces multi-cluster routing capability that allows a broker to query both local and remote Pinot clusters, enabling federated query execution across cluster boundaries. Subsequent PRs will hook these routing managers with the larger federation flow.

Changes

• Added MultiClusterRoutingManager - orchestrates routing across local and remote Pinot clusters
• Added RemoteClusterBrokerRoutingManager - manages routing for individual remote clusters with periodic sync
• Modified BrokerRoutingManager visibility to support inheritance (5 fields/methods: private → protected)
• Added REMOTE_CLUSTER_BROKER_ROUTING_CALCULATION_TIME_MS metric to BrokerTimer

These are new classes which are not hooked to any production flow currently, and hence do not impact any regular functionality.

Testing

Tested as a part of the draft PR: https://github.com/apache/pinot/pull/17145. Added unit tests in this PR and integration tests will be added subsequently.

[codecov-commenter]

Codecov Report

:x: Patch coverage is 81.25000% with 24 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 63.29%. Comparing base (8fd34ec) to head (d89d815). :warning: Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
...not/broker/routing/MultiClusterRoutingManager.java	78.75%	12 Missing and 5 partials :warning:
...ker/routing/RemoteClusterBrokerRoutingManager.java	84.44%	4 Missing and 3 partials :warning:

Additional details and impacted files

@@             Coverage Diff              @@
##             master   #17296      +/-   ##
============================================
+ Coverage     63.22%   63.29%   +0.07%     
  Complexity     1474     1474              
============================================
  Files          3151     3154       +3     
  Lines        187851   187996     +145     
  Branches      28760    28779      +19     
============================================
+ Hits         118767   118991     +224     
+ Misses        59868    59787      -81     
- Partials       9216     9218       +2     

Flag	Coverage Δ
custom-integration1	100.00% <ø> (ø)
integration	100.00% <ø> (ø)
integration1	100.00% <ø> (ø)
integration2	0.00% <ø> (ø)
java-11	63.24% <81.25%> (+7.60%)	:arrow_up:
java-21	63.23% <81.25%> (+0.02%)	:arrow_up:
temurin	63.29% <81.25%> (+0.07%)	:arrow_up:
unittests	63.29% <81.25%> (+0.07%)	:arrow_up:
unittests1	55.68% <100.00%> (+0.02%)	:arrow_up:
unittests2	33.94% <81.25%> (+0.07%)	:arrow_up:

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