Add SDL/macro type extensions

[maartenvanvliet]

Gave it a shot to add support for extend to SDL. Once that was running, adding macro support was not that difficult.
It follows the graphql spec for the most part, though schema declarations cannot be extended as of yet. The validations are done by the currently existing validation phases.

I looked at #821 but decided against the

extend :foo do
   field :name, :string
end

syntax in favor of

extend do 
   object :foo do
      field :name, :string
   end
end

since it was easier to implement and is closer to the graphql syntax. I also think it will work better with imports.

Imports across modules work as well, using the type_extensions field. It uses the TypeImports phase, though it could be moved to a different module if necessary or even a separate import_type_extensions macro instead of the import_types macro.

Fixes #772

[kdawgwilk]

Could this support this syntax? Since that is what absinthe_relay uses and may already be familiar to consumers of absinthe https://hexdocs.pm/absinthe_relay/Absinthe.Relay.Node.html#module-object

extend object :foo do
    field :name, :string
end

absinthe_relay

node object :person do
  field :name, :string
  field :age, :string
end

[maartenvanvliet]

Ah interesting, forgot about that syntax usage in relay. I'll have a look.

[maartenvanvliet]

Thx! Fixed it in 675808a

[maartenvanvliet]

The formatting failures in CI were pre-existing

[benwilson512]

Excellent stuff, can you add a changelog entry?

[maartenvanvliet]

Thx! I've added an entry 🎉

[kdawgwilk]

Do you still need this in this phase now that you added that other ApplyTypeExtensions phase?

[maartenvanvliet]

I've extracted the type extension imports to a separate phase, like with import_directives.

[maartenvanvliet]

Changed the import to use import_type_extensions analogous to import_types and import_directives.

Also, added a fix to extend builtin types through type extensions and changed the DeprecatedDirectiveFields phase to use that.

[maartenvanvliet]

Moved the defaults to the pipeline configuration. This way the DeprecatedDirectiveFields phase is no longer needed as the same can be achieved with configuration.

@kdawgwilk sorry, this clashes a bit with your PR. The same structure can be applied there.

[benwilson512]

@maartenvanvliet What is the overall state of this PR?

[maartenvanvliet]

@maartenvanvliet What is the overall state of this PR?

Just fixed a merge conflict with the changelog. Now, it's good to go. 🚀

I have an idea around extending scalar types, so a type extension can be used to make the Int type spec compliant (see #921) It goes beyond the spec and can be left for another PR though.