add uuid type

[it512]

A Universally Unique Identifier (UUID) is a 128-bit used for information in computer systems. The term Globally Unique Identifier (GUID) is also used, mostly in Microsoft systems.

Because Go (still) does not have built-in support for UUID types, module google/uuid is imported. It's one of the most complete, correct (and starred) modules nowadays (the other being github.com/gofrs/uuid.UUID).

This pull request introduces support for Universally Unique Identifiers (UUID) within gqlgen.

Many people may have already chosen a different UUID library:

models:
  UUID:
    model:
      - github.com/gofrs/uuid.UUID

[coveralls]

coverage: 75.788% (+0.01%) from 75.777% when pulling 6fb1f49 on it512:master into 2d8673a on 99designs:master.

[StevenACoffman]

Ok, so this is just like PR #2749 but here the choice is github.com/google/uuid instead of there it is github.com/gofrs/uuid

Many people will have already chosen one of those two (or something else) and so will have:

models:
  UUID:
    model:
      - github.com/gofrs/uuid.UUID

Please add an example that uses this to verify that people can still override the default choice.

[it512]

UUID type

add to gqlgen.yalm

models:
  UUID:
    model:
      - github.com/99designs/gqlgen/graphql.UUID

add UUID scalar to schema.graphqls

scalar UUID

See the _examples/uuid package for more examples.

[it512]

add example in _examples/uuid and modify scalar doc (docs/content/reference/scalar.md)

[it512]

ok， thank you！

[it512]

Is there anything else that needs to be improved or revised?

[StevenACoffman]

Thanks for your patience. I was quite ill for a while, and I'm only now getting caught up at my day job so I can return to gqlgen. I added a commit that used the documentation from #2749 here (thanks!) and I'm merging this one.

[0x221A]

Is it better to return the null value instead of the zero value uuid?