Extension: Use context managers for lifecycle hooks

[ThirVondukr]

Feature Request Type

• Core functionality
• Alteration (enhancement/optimization) of existing feature(s)
• New behavior

Description

Introduce a new interface for extension hooks - on request/validation/parsing/execution using context managers instead of on start/end hooks:

def on_request(self) -> Iterable[None]:
    #  This part is called when a GraphQL request starts
    yield
    # This part is called when a GraphQL request ends

This makes it easier to manage resources (e.g. open connections) and share data between start/end hooks