Support extending a consumer method using the definition of another

[prkumar]

Is your feature request related to a problem? Please describe.
This issue is spun out of a discussion on Gitter with @liiight:

@prkumar i have a question. I wanna do something like this:

class Lestrade(FlapiBase):
    @lestrade_error
    @json
    @post('lestrade/graphql')
    def graphql(self, **body: Body):
        pass

    @returns.json(member=('data', 'incidents'))
    def get_incident(self, **body) -> Incident:
        return self.graphql(**body)

    @returns.json(member=('data', 'updateIncident'))
    def update_incident(self, **body: Body) -> UpdateCommandResponse:
        return self.graphql(**body)

Essentially, we want two separate "child" consumer methods that "inherit" details from the graphql consumer method.

Describe the solution you'd like

In summary, we can use the graphql consumer method as a decorator on subsequent methods in the class definition so that they can inherit and enhance the request definition, separately. (This should be actually pretty simple to implement.)

class Lestrade(FlapiBase):
    @lestrade_error
    @json
    @post('lestrade/graphql')
    def graphql(self, **body: Body):
        pass

    @returns.json(member=('data', 'incidents'))
    @graphql
    def get_incident(self, **body) -> Incident:
        pass

    @returns.json(member=('data', 'updateIncident'))
    @graphql
    def update_incident(self, **body) -> UpdateCommandResponse:
        pass

Notably, the decorated methods need to have the same argument signature as the graphql method.