Allow using methods as Rpc targets instead of needing to define call.

[just-be-dev]

I find the typescript usage of RpcTarget to be significantly more ergonomic.

class MyApiServer extends RpcTarget {
  hello(name) {
    return `Hello, ${name}!`
  }
}

In their usage, all you really need to do is extend from RpcTarget and define some methods. Every non-private method is then callable from the RPC connection. I thought it'd be nice to do the same thing here. So instead this

class MyApiServer(RpcTarget):
  async def call(self, method: str, args: list[Any]) -> Any:
    match method:
      case "hello":
        return f"Hello, {args[0]}"

we could just implement it as

class MyApiServer(RpcTarget):
  def hello(self, name: str) -> str:
    return f"Hello, {name}"

The nice thing about my implementation approach is that if you already have a definition of call, it'll continue to work exactly the same as it does today. You just are no longer required to implement call because there's a default implementation.

---

There's a few longer term things that I think would be nice to do

1. Make the client interface seem more like it's a regular object (instead of the call method)
2. Make the target Pydantic compatible to ensure inputs/outputs can be validated