Add support to send variables for GraphQL queries

rafasfrequested to merge
gitlab-community/gitlab-org/api/client-go:rafasf-graphqlvars into main

It extends GraphQLQuery with the Variables to allow consumers to leverage the built-in support for those when using the GraphQL interface.

No changes were necessary on the actual request execution as it marshals the any and /graphql has support for the both payloads.

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