Use godoc for descriptions

[shreyas44]

Currently, to provide descriptions for fields we need to use the description struct tag. While this is fine, the lines would be really long for long descriptions, and it might be easier to use regular docstrings for descriptions.

Instead of

type User struct {
  ID string `description:"user id"`
}

We do

type User struct {
  // user id
  ID string
}

Using docstrings would also allow us to support descriptions for enums and type definitions. We should continue to support the description struct tag as an override in case the user wants to use different descriptions on the GraphQL and Go sides.

[shreyas44]

Just realised this isn't possible since there's no way to get the descriptions at runtime without creating external dependencies.

[shreyas44]

We could use the json file generated by go doc for this - depends on golang/go#34293