Docs for request lifecycle end hooks and responseForOperation

[nwalters512]

This PR builds on #2008 and adds documentation for request lifecycle event end hooks and the new responseForOperation event.

[apollo-cla]

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[abernix]

Thanks so much!

[DoctorVoid]

@nwalters512
Can someone show how can I override the response promise by using responseForOperation?

It seems that if I try to return anything other than null from this hook, It results in a weird infinite loop inside the request pipeline.

This results in a never ending request (regardless if its an async promise or what type or result):

public responseForOperation(
        requestContext: GraphQLRequestContext<PolarisGraphQLContext> &
            Required<Pick<GraphQLRequestContext<PolarisGraphQLContext>,
                'metrics' | 'source' | 'document' | 'operationName' | 'operation'>>,
    ): Promise<GraphQLResponse | null> | GraphQLResponse | null {
        return new Promise(async () => {
            const result = await this.someOtherRequest()
            return result;
        });
    }

This is fine, but doesn't overrides the original request

public responseForOperation(
        requestContext: GraphQLRequestContext<PolarisGraphQLContext> &
            Required<Pick<GraphQLRequestContext<PolarisGraphQLContext>,
                'metrics' | 'source' | 'document' | 'operationName' | 'operation'>>,
    ): Promise<GraphQLResponse | null> | GraphQLResponse | null {
        return null;
    }