apollo_gateway: stateful_validator_factory::instantiate_validator to async

[itamar-starkware]

TL;DR

Made instantiate_validator method async in the StatefulTransactionValidatorFactoryTrait.

What changed?

• Changed the instantiate_validator method in StatefulTransactionValidatorFactoryTrait to be async by adding the async keyword and using #[async_trait]
• Updated all implementations and calls to this method to use .await
• Modified test functions to support the async nature of the validator instantiation
• Updated the process_tx_task function in tests to be async

How to test?

• Run the existing test suite to ensure all tests pass with the async implementation
• Verify that the gateway can still instantiate validators correctly
• Check that the async flow works properly in the transaction processing pipeline

Why make this change?

This change allows for asynchronous initialization of transaction validators, which can improve performance by not blocking the execution thread during validator instantiation. This is particularly important for operations that might involve I/O or other potentially blocking operations during validator setup.

[reviewable-StarkWare]

This change is 

[itamar-starkware]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• apollo_gateway: move get_block_info to async #10060
• apollo_gateway: move get_state_reader_from_latest_block to async #10059
• apollo_gateway: stateful_validator_factory::instantiate_validator to async #10058 👈 (View in Graphite)
• apollo_gateway: change ProcessTxBlockingTask::new to be async #10057
• main-v0.14.1

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