[Request] Add missing RPC Client constructors (with commitment, with timeout)

[qsish]

The rpc.Client is missing several constructor variants that the Rust SDK provides out of the box. The most painful gap is the inability to set a default commitment on the client — every call that needs a CommitmentType has to pass it explicitly. The HTTP timeout is also hardcoded and cannot be configured without assembling a JSONRPCClient by hand.

Proposal: add missing constructors

• NewWithCommitment(url, commitment) — store a default commitment on the Client so every call that needs one uses it without having to pass it explicitly.
• NewWithTimeout(url, timeout) — make the HTTP timeout configurable (currently hardcoded to 5 minutes).
• NewWithTimeoutAndCommitment(url, timeout, commitment) — combined variant, matching the Rust SDK.

[sonicfromnewyoke]

good point, im working right now on rpc-client parity and will "mimic" the API.

this will be a breaking change, so likely it will be a part of v2