feat: load gqlgen plugins during parse schema

[giautm]

Currently, the EntGQL extension didn't load any GQLGen plugins. So, When I use Federations (it's a built-in GQLGen plugin), EntGQL complains about undefined @key directives which will be defined by the plugin via code-mod.

# Todo with Federations enabled
type Todo implements Node @key(fields: "id") {
  id: ID!
  createdAt: Time
  status: Status!
  priority: Int!
  text: String!
  parent: Todo
  children: [Todo!]
}

Current error

2022/01/05 10:43:17 creating EntGQL extension: internal/graphql/schema/schema.gql:54: Undefined directive key.
exit status 1

This PR loads built-in plugins from GQLGen, and also provides options to load third-party plugins.

[masseelch]

Can you add tests for this?

[giautm]

What kind of test? 😁 trying to enable federation?!

[giautm]

Hey @masseelch, plz review again. 🥰

[masseelch]

This is what I meant yes. Though I advise to let todo, todopulid and todouuid unchanged and simply add another dir liketodofed or similar. Looks great otherwise 👍

And we need to see if @a8m is willing to merge it :-)

[giautm]

Updated PR with todofed, and unchanges other apps.

[masseelch]

Thank you :-)

[a8m]

Looks good 🚀

Add minor style comments and a few thoughts.

[a8m]

Thanks for the contribution @giautm ❤️