Adding support for batching JSON-RPC requests.

Batching JSON-RPC request is an efficient way to reduce the overhead of HTTP communication.

The batching syntax are documented in https://www.quicknode.com/guides/ethereum-development/transactions/how-to-make-batch-requests-on-ethereum

An example batching implementation can be found https://github.com/hyperledger/firefly-transaction-manager/blob/1ac00b5eb37664d73abebd8822ae8439cef3f4bc/internal/persistence/postgres/transaction_writer.go#L79

I think we should use this chance to pull out the batching logic as a library in firefly-common as well.

Start by reviewing the batching implementation in internal/persistence/postgres/transaction_writer.go and the linked JSON-RPC batching documentation. Determine the intended reusable library boundary in firefly-common, then define completion as signer supporting batched JSON-RPC requests without duplicating batching logic.

Written by the indexing model from the issue text.