gqlgen generates non compilable code when schema has Entity interfaces

### What happened?

gqlgen generates non compilable code when schema has [Entity interfaces](https://www.apollographql.com/docs/federation/federated-types/interfaces/).

I've noticed this after trying to upgrade gqlgen from `v0.17.31` -> `v0.17.34`

### What did you expect?

It should generate compilable code

### Minimal graphql.schema and models to reproduce

```graphql
interface User @key(fields: "id") {
id: ID!
name: String!
email: String!
}

type Customer implements User @key(fields: "id") {
id: ID!
name: String!
email: String!
}

extend type Query {
me: User!
}
```
gqlgen is generating this model for the interface, but it doesn't implement the `fedruntime.Entity` interface.

```go
type User interface {
IsUser()
GetID() string
GetName() string
GetEmail() string
}
```

### versions
- `go run github.com/99designs/gqlgen version`? `v0.17.34`
- `go version go1.20.5 darwin/arm64`

问题出在使用 Apollo Federation 的 @key 指令生成 GraphQL 接口的代码生成逻辑中。查看 gqlgen 的代码生成逻辑，可能位于处理接口的插件生成或模型生成包中。生成的 User 接口还必须实现 fedruntime.Entity。检查现有的 federation 或实体生成测试，以了解预期输出，并为此 schema 添加一个测试用例。

由索引模型根据 Issue 内容生成。