Generate subscriptions as async generators

[mat-sop]

resolves #65
Example of generated client:

class Client(AsyncBaseClient):
    async def get_counter(self) -> AsyncIterator[GetCounter]:
        query = gql(
            """
            subscription getCounter {
              counter
            }
            """
        )
        variables: dict[str, object] = {}
        async for data in self.execute_ws(query=query, variables=variables):
            yield GetCounter.parse_obj(data)

[rafalp]

Make this dependency opt-in, eg. pip install ariadne-codegen[subscriptions]

[mat-sop]

Hm I'm not sure if it's the right way. AsyncBaseClient still depends on websockets even when no subscription was generated.

[rafalp]

It should handle websockets not being installed. Usual approach is handling the import error and importing placeholder object that implements the subset of API but raises not implemented error from its methods.

[rafalp]

Suggested change

	For every provided query/mutation/subscription there is generated file, that contains models which correspond to return type of operation. File name is generated by converting operation name to snake case. Root models has the same name as query/mutation/subscription, and depend classes use it as prefix in their names.
	For every provided query/mutation/subscription there is a generated file that contains models which correspond to return type of operation. File name is generated by converting operation name to snake case. Root models has the same name as query/mutation/subscription, and depend classes use it as prefix in their names.

[rafalp]

We could group message types under an Enum

[rafalp]

We should mention in readme that websockets are optional dependency required for subscriptions.