Add support for operationName in API.graphql

[TuckerWhitehouse]

Is your feature request related to a problem? Please describe.
Using API.graphql with GraphiQL does not work with multiple queries in the editor.

Describe the solution you'd like
API.graphql accepts query and variables, it would be great to have it accept operationName as well.

Describe alternatives you've considered
I think it's possible to manually construct a fetch request with the appropriate headers, but that forgoes using the amplify api packages entirely.

Additional context
Named operations and passing an operationName are part of the GraphQL specification
http://spec.graphql.org/June2018/#sec-Executing-Requests

[TuckerWhitehouse]

WIP Branch started here: aws-amplify/amplify-js@main...TuckerWhitehouse:api-graphql/feat/operation-name

[bengiddins]

With the release of AWS_LAMBDA as an authorization method for AppSync, I would also like to see this operationName available. Requests direct from the AppSync console return a populated operationName variable in the request context to Lambda, however calls to the same AppSync API from Amplify result in an empty operationName with no way of setting it.