Expose explicit WebSocket URL constructors; preserve default new behavior

[Royc30ne]

Description:

• Adds a public constructor to allow providing an explicit WebSocket PubSub URL: DriftClient::new_with_ws_url(...).
• Keeps the existing DriftClient::new(...) for backward compatibility; it now delegates to the explicit constructor to reuse validation/initialization logic.
• Adds DriftClientBackend::new_with_ws_url (and internal new_with_explicit_ws_url) to initialize the backend with a custom wss://host:port endpoint.
• Validates both RPC and WS URLs early with get_http_url / get_ws_url to fail fast on malformed endpoints.
• Updates relevant docs/comments in crates/src/lib.rs.

Why: Some users need to connect to a custom/hosted Ws PubSub endpoint (e.g. private relay or proxy) rather than deriving from the RPC URL. Exposing an explicit WS constructor makes that supported while preserving the default behavior for existing callers.

Files changed:

• crates/src/lib.rs (public API + backend constructors and docs)

Migration:

• No breaking changes. Existing code using DriftClient::new(...) continues to work.
• To use a custom WS endpoint, call:
  DriftClient::new_with_ws_url(context, rpc_client, wallet, "wss://host:port").await?;

Testing: Verified compilation and local tests.

[jordy25519]

hi @Royc30ne needs a format but changes look ok to me apart from that

[Royc30ne]

@jordy25519 ran cargo fmt and pushed the updated formatting. Let me know if anything else needs tweaking!