feat: Add Client::call_async

[ValuedMammal]

Description

This PR adds Client::call_async, an async counterpart to the existing Client::call method. It mirrors the sync implementation, by building a JSON-RPC Request with an auto-incremented ID, invoking a caller-supplied send_fn to perform the transport, verifying the response ID matches the request ID, and deserializing the result into T, but accepts an AsyncFn(&Request) -> Result<Response, E> and awaits the response instead of calling a blocking closure. Since Client remains sans-io, this lets consumers plug in an async transport without needing a parallel async client type or duplicating request/response handling logic.

Notes to the reviewers

call_async's send_fn closure takes the jsonrpc::Request by reference (&Request) rather than by value like the sync call. This is for ergonomic reasons, as serializing to JSON using serde_json is typically non-consuming. Error handling, ID validation, and serialization logic are otherwise identical to call.

Changelog notice

• Added Client::call_async for making JSON-RPC calls with an async send_fn transport.

Checklists

All Submissions:

• I've signed all my commits
• I ran just pre-push before committing
• I followed the contribution guidelines

New Features:

• I've added tests for the new feature
• I've added docs for the new feature

[tvpeter]

Thank you for working on this.

The implementation is sound and good to go.

I left a nit.

[ValuedMammal]

Rather than pull in these dev-dependencies right now, we could add Client::call_async in a separate commit (23c6fd6) and leave the async example as a draft.

[ValuedMammal]

I may have figured out a cleaner solution to call_async by simplifying the send_fn at the callsite.

• Still to do: Reword the commit and PR description (48901da) ✅

[tvpeter]

ACK 48901da

I left a non-blocking nit.

[tvpeter]

reACK 4a99a54