[Enhancement⚡️] Add broker handler for GetAllDelayOffset to expose delay offset data #3906

Summary
Expose broker-side admin handling for GetAllDelayOffset by wiring the request into AdminBrokerProcessor and implementing OffsetRequestHandler::get_all_delay_offset to return encoded delay offset content.

Motivation

• Provide an administrative endpoint to retrieve all delay offsets from the broker, useful for monitoring, diagnostics, and slave synchronization steps.

Scope of Changes (files)

• Modified: rocketmq-broker/src/processor/admin_broker_processor.rs
  • Route RequestCode::GetAllDelayOffset to offset_request_handler.
• Modified: rocketmq-broker/src/processor/admin_broker_processor/offset_request_handler.rs
  • Implement get_all_delay_offset: encodes schedule message service state and returns it in response body; returns error if empty.

Behavior/Details

• When invoked with GetAllDelayOffset, broker responds with the serialized delay offset content via schedule_message_service.encode_pretty(false).
• If there is no delay offset content, returns SystemError with an explanatory remark.

Compatibility/Notes

• Adds a new handled request type; does not break existing behavior.

Follow-ups

• Consider adding unit/integration tests for GetAllDelayOffset response formatting and error path.
• Wire client-side helper in BrokerOuterAPI (if needed) to call this admin endpoint.