Implement bulk authorization methods in KeycloakAuthManager

[abhishekmauryaKsolves]

Closes #70582

Implements the missing batch/filter authorization methods in
KeycloakAuthManager, following the existing pattern used by
filter_authorized_dag_ids (parallel HTTP calls via ThreadPoolExecutor,
with caching via single_flight).

• batch_is_authorized_connection
• batch_is_authorized_dag
• batch_is_authorized_pool
• batch_is_authorized_variable
• filter_authorized_connections
• filter_authorized_pools
• filter_authorized_variables

Tests added covering normal filtering, empty input, all-denied, and
cache-hit scenarios for each new filter method.

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[stephen-bracken]

This implementation matches the implementation in filter_authorized_dag_ids, which is good for consistency, but I think we should utilise the _is_batch_authorized method to streamline the checks here, and avoid calling keycloak N times for N resources