[proc macros/jsonrpsee]: less deps when defining client API only

[niklasad1]

Currently in the wrapper/facade crate jsonrpsee it's not possible to just import some minor deps for the client traits.
If I want to something like:

use jsonrpsee::{core::RpcResult, proc_macros::rpc};
#[rpc(client)]
pub trait Api {
	#[method(name = "call")]
	fn call(&self, x: usize) -> RpcResult<()>;
}

When trying to do that one must either enable async-client feature which brings a few needless deps including tokio

It's also possible though to import jsonrpsee-core/client manually to workaround that but we should probably expose a feature for that.

[niklasad1]

Closed by #849