chainHead: Add support for storage pagination and cancellation

[lexnv]

This PR adds configurable pagination for the storage query types: DescendantHashes and DescendantValues with the possibility to cancel the operation after the WaitingForContinue event is generated.

• implement chainHead_unstable_continue method
• implement chainHead_unstable_stopOperation method
• add configurable pagination limit after which the WaitingForContinue event is generated
• iterable storage queries are placed in a queue for providing fairness across multiple queries of the same operation
• limiter object from https://github.com/paritytech/substrate/pull/14699 is refactored inside an Operations object for better ergonomics
• registering an operation shares a lock-free state between the backend and frontend for efficient signaling of continue or stop
  • RegisteredOperation object is returned to the backend (chainHead event generation) via the BlockGuard
  • OperationState object is returned to the frontend and has direct user access via chainHead_unstable_continue and chainHead_unstable_stopOperation
• RegisteredOperation in RAII manner (on drop) releases the capacity permit for the operation and unregisters the operation from the subscription state
• integration-tests for the continue and stop operations (via check_continue_operation and stop_storage_operation)

Builds on top of: https://github.com/paritytech/substrate/pull/14699 Closes: https://github.com/paritytech/substrate/issues/14549 Closes: https://github.com/paritytech/substrate/issues/14639

// @paritytech/subxt-team