Upgrade the usage of the client to make it more convenient for users.

[gpBlockchain]

Currently, the Rust SDK hardcodes the client as reqwest::blocking::Client::new().

ckb-sdk-rust/src/rpc/mod.rs

Line 45 in 563161a

$struct_name { url, id: 0.into(), client: reqwest::blocking::Client::new(), }

However, sometimes users may need to customize the client, for example:

• They may want an asynchronous client.
• They may want to set retry options or timeouts.
  example :
  https://github.com/godwokenrises/godwoken/blob/dbb3c1bf9a4593bae078428d725f7372c2f5401f/crates/rpc-client/src/ckb_client.rs#L12

https://github.com/synapseweb3/forcerelay/blob/07c3463d2200423740366f5f9b0440bdbc17618a/crates/relayer/src/chain/ckb/rpc_client.rs#L72-L81