Add a plugin for configuring gqlgen via directives

[lwc]

Uses the plugin system / MutateConfig interface to allow for gqlgen configuration directly in the schema files, via two directives:

directive @goModel(model: String, models: [String!]) on OBJECT 
    | INPUT_OBJECT 
    | SCALAR 
    | ENUM 
    | INTERFACE 
    | UNION

directive @goField(forceResolver: Boolean, name: String) on INPUT_FIELD_DEFINITION 
    | FIELD_DEFINITION

I think this captures all the features available in gqlgen.yml.

Benefits

• Config lives alongside the schema
• Allows config to be split across multiple schema files

Questions

• Naming is hard - originally I'd called these bind and field but was concerned they were too vague. Thoughts?

Supersedes: #727
Fixes: #587, #726

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

I'll update docs if we decide to go ahead with this approach

[sunfmin]

make these directives builtin? Or better not?

[coveralls]

Coverage decreased (-0.8%) to 61.896% when pulling c14f865 on schemaconfig-plugin into 5e7e94c on master.