[improve][broker] Add configuration to enable shadow topics - #26739

Motivation

Brokers already provide enablePersistentTopics and enableNonPersistentTopics to control which topic types they support. Add an equivalent setting for shadow topics so operators can explicitly choose whether to enable the feature.

Modifications

• Add enableShadowTopics=false to the broker configuration and the broker/standalone configuration files. Changing the setting requires a broker restart.
• Apply the setting to shadow topic creation, shadow-source property updates, topic loading, and replication. Policy inspection and removal remain available when disabled.
• Use the existing tenant and superuser access rules when validating shadow-topic configuration, with asynchronous checks.
• Update existing tests to enable the feature explicitly and cover both enabled and disabled behavior for partitioned and non-partitioned topics.

Deployments using shadow topics must set enableShadowTopics=true on their brokers to retain that behavior.

Verifying this change

• ./gradlew quickCheck and ./gradlew sanityCheck passed.
• 26 focused tests passed, covering topic configuration, loading, replication, and access rules.
• Configuration-loader and service-configuration test suites passed.
• Make sure that the change passes the CI checks.

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

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations — shadow topics require explicit enablement.
• The threading model
• The binary protocol
• The REST endpoints — shadow-topic configuration follows the feature setting and access rules.
• The admin CLI options
• The metrics
• Anything that affects deployment — existing shadow-topic deployments must enable the feature and restart brokers.