Allow async execution of queries in client

[DCNick3]

We should a mechanism that would allow query-related HTTP requests to be send asynchronously. For example, one can extend QueryExecutor trait with QueryExecutorAsync trait and add an async version of QueryIterator.

This is needed for iroha blockchain explorer, which currently implements its own async-capable query builder.

Originally raised in https://github.com/hyperledger/iroha/pull/4833#issuecomment-2268342102.