doc: add documentation for Security dependencies

[adriangb]

Make sure to include the ability to dynamically configure security dependencies in lifespan events:

xpresso/tests/test_security/test_security_api_key_header_deferred.py

Lines 12 to 34 in 88840e9

	def get_name() -> str:
	return "key"
	class LazyAPIKeyHeader(APIKeyHeader):
	def __init__(self, name: Annotated[str, Dependant(get_name, scope="app")]) -> None:
	super().__init__(name=name)
	class User(BaseModel):
	username: str
	def get_current_user(oauth_header: Annotated[str, Security(LazyAPIKeyHeader)]):
	user = User(username=oauth_header)
	return user
	def read_current_user(current_user: Annotated[User, Dependant(get_current_user)]):
	return current_user
	app = App([Path("/users/me", get=read_current_user)])