Config option for generating embedded structs for GraphQL interfaces

[ianling]

See discussion #2212 for more context.

Given the following GraphQL schema:

interface Animal {
    species: String!
}

type Human implements Animal {
    species: String!
    name: String!
}

type Dog implements Animal {
    species: String!
    breed: String!
}

gqlgen without this PR generates the following Go code:

type Animal interface {
	IsAnimal()
}

type Human struct {
	Species string `json:"species"`
	Name    string `json:"name"`
}

func (Human) IsAnimal() {}

type Dog struct {
	Species string `json:"species"`
	Breed    string `json:"breed"`
}

func (Dog) IsAnimal() {}

gqlgen with this PR and a config that includes generate_embedded_structs_for_interfaces: true generates the following Go code (existing users that do not specifically opt in to the new behavior by adding this option to their config will continue getting the same behavior as before this PR):

type Animal struct {
	Species string `json:"species"`
}

type Human struct {
	Animal
	Name    string `json:"name"`
}

type Dog struct {
	Animal
	Breed    string `json:"breed"`
}

I have:

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

[ianling]

Note: this commit makes uses of continue/break statements within the modelgen Go template. This was a feature added in Go 1.18.

[StevenACoffman]

Sorry, but we can't move to Go 1.18 yet. We are stuck on Go 1.16(!) until GCP AppEngine supports a newer version (3-6 months), and I don't feel comfortable maintaining this library when I can't vet it against our production use cases.

[ianling]

Yeaaaah, my bad, I didn't realize that break/continue in Templates was a 1.18 feature until like a week after I'd written it. I might be able to come up with an alternate solution in the meantime. 

[neptoess]

@ianling
My apologies, but I had a couple PRs (#2314 #2315) get merged recently that almost certainly caused this one to have conflicts. Would recommend rebasing on the latest master.

[jgalliers]

We are encountering this in our software and this seems like a good quality-of-life enhancement. Can this be progressed? 🤞