Interface implementations are missing

[HitkoDev]

Describe the Bug
Interface implementations are stripped by

type-graphql/src/schema/schema-generator.ts

Lines 591 to 593 in cd90856

	if (!this.usedInterfaceTypes.has(interfaceClass)) {
	return false;
	}

unless interface is directly used in @Mutation or @Query decorators.

To Reproduce

import { printSchema } from "graphql";
import "reflect-metadata";
import { buildSchema, Field, InterfaceType, ObjectType, Query, Resolver } from "type-graphql";

@InterfaceType()
export class IFace {
    @Field(() => String)
    val: string;
}

@ObjectType({ implements: IFace })
export class Foo extends IFace {
    constructor() {
        super();
        this.val = "Foo";
    }
}

@ObjectType({ implements: IFace })
export class Bar extends IFace {
    constructor() {
        super();
        this.val = "Bar";
    }
}

@ObjectType()
export class FooBar {
    @Field(() => IFace)
    iface: IFace;

    constructor() {
        this.iface = Math.random() > 0.5
            ? new Foo()
            : new Bar();
    }
}

@Resolver()
export class QueryResolver {
    // Doesn't include Foo & Bar
    @Query(() => FooBar)
    foobar() {
        return new FooBar();
    }

    // Properly includes Foo & Bar
    /* @Query(() => IFace)
    iface() {
        return Math.random() > 0.5
            ? new Foo()
            : new Bar();
    }*/
}

buildSchema({
    resolvers: [
        QueryResolver,
    ],
})
    .then(schema => console.log(printSchema(schema)))
    .catch(console.error);

Actual Behavior
Generated schema looks like

type Query {
  foobar: FooBar!
}

type FooBar {
  iface: IFace!
}

interface IFace {
  val: String!
}

Expected Behavior
Generated schema looks like

type Foo implements IFace {
  val: String!
}

interface IFace {
  val: String!
}

type Bar implements IFace {
  val: String!
}

type Query {
  foobar: FooBar!
}

type FooBar {
  iface: IFace!
}

Logs
Runtime Object type "Foo" is not a possible type for "IFace".

Environment (please complete the following information):

• OS: all
• Node: all
• Package version: since 1.0.0
• TypeScript version: all

Additional Context
It apereas that usedInterfaceTypes in schema-generator.ts is only properly populated after all field thunks are resolved, which doesn't happen until after we call new GraphQLSchema in

type-graphql/src/schema/schema-generator.ts

Line 132 in cd90856

const schema = new GraphQLSchema({

[MichalLytek]

The docs says:

By default, if the interface type is explicitly used in schema definition (used as a return type of a query/mutation or as some field type), all object types that implement that interface will be emitted in schema, so we don't need to do anything.

So it looks like a bug, I'll have to cover your case by tests.
In the meantime you can use orphanedTypes as a workaround for now.