[Feat]: Support adding new RPC methods to a JSON-RPC server application

### Is your feature request related to a problem? Please describe.

The spec for [extensions](https://a2a-protocol.org/latest/topics/extensions) defines 3 types of extensions, the last one is: _"Method Extensions (Extended Skills): Adding entirely new RPC methods beyond the core set defined by the protocol"_
As far as I understand, there is currently no way for enhancing my `A2AStarletteApplication`/`A2AFastAPIApplication` application with new RPC methods. (as they both extends `JSONRPCApplication` and it uses a fixed list of the core RPC methods that cant be extended)

### Describe the solution you'd like

I would like `JSONRPCApplication` to allow its users to hook new JSON-RPC methods.

### Describe alternatives you've considered

I'm considering using the HTTP/REST protocol instead of the JSON-RPC protocol just because its easier to extend with new methods.

### Additional context

_No response_

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