[improve][broker] Support namespace-level configuration of migratedClusterUrl in blue-green migration feature

[hrzzzz]

PIP: #23790

Motivation

Currently, Pulsar supports blue-green cluster migration at both the cluster and namespace levels, but the migratedClusterUrl can only be set at the cluster level, meaning that migration can only occur to a single cluster.

However, there are times when we want to migrate different namespaces to different clusters. For example, a certain namespace belongs to a specific business and is currently in a public cluster. We want to migrate this namespace to a dedicated cluster for that business. Therefore, we would like to add the migratedClusterUrl configuration at the namespace level to support this functionality

Modifications

Allow specifying migratedClusterUrl at the namespace level.

Verifying this change

• Make sure that the change passes the CI checks.

Add test in: org.apache.pulsar.broker.service.ClusterMigrationTest#testNamespaceMigrationWithNamespaceLevelMigratedClusterUrl

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete

Matching PR in forked repository

PR in forked repository: hrzzzz#15