Add GraphQL support for Go backend application

[bmonkman]

We'd like to make it easy for people to spin up a new application with GraphQL support.

Probably using https://github.com/99designs/gqlgen which has a nice schema-first approach. This would also require an extra step in the build process to generate code from the schema.

There should be some example code we can probably reuse in https://github.com/commitdev/go-gqlgen-skeleton

See also similar support in the node backend. The example schemas of these two projects must be the same.
zero-deployable-node-backend#12

Keep in mind that we will want to support multiple API types including both GraphQL and generating the API using OpenAPI/Swagger. Try to implement this in such a way that we can turn it off / swap between different API types based on user choice.