feat: Support sub-extensions with custom hooks

[pawamoy]

This PR would allow users to define custom events, for their own downstream users to implement the corresponding hooks. It's an extension system in the extension system. Inspired by mkdocs/mkdocs#3806.

from ast import AST
from typing import Any

import griffe


##############################################
# Extension writer.
class CustomEvents(griffe.SubExtension):
    # By default, the namespace is the class name.
    namespace = "my_sub_extension"

    def on_whatever(self, param1: str, **kwargs) -> None:
        ...


class SomeExtension(griffe.Extension):
    def on_node(self, agent: griffe.Visitor | griffe.Inspector, **kwargs: Any) -> None:
        # Fire the event, calling hooks declared by downstream users.
        agent.extensions.subcall(CustomEvents.namespace, "on_whatever", param1="Hello!", extra="friends")


##############################################
# User hooks.
class MyHooks(CustomEvents):
    def on_whatever(self, param1: str, **kwargs) -> None:
        print(param1, kwargs)


class SomeRelatedExtension(griffe.Extension):
    sub_extensions = (MyHooks(),)

    # Or declare them in `__init__`, allowing further customization through options.
    def __init__(self, **options) -> None:
        self.sub_extensions = (MyHooks(**options),)


##############################################
# Final user.
extensions = griffe.load_extensions(SomeExtension, SomeRelatedExtension)
...

[pawamoy]

Closing for now, we can revisit if someone asks for it.