fix(federation): override federated resolveType for interfaces

[thomas-advantitge]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Interfaces example (https://docs.nestjs.com/graphql/interfaces) with resolveType doesn't work when changing GraphQLModule to GraphQLFederationModule.

{
  "errors": [
    {
      "message": "Abstract type \"Party\" must resolve to an Object type at runtime for field \"Query.parties\". Either the \"Party\" type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function.",
      "locations": [
        {
          "line": 2,
          "column": 3
        }
      ],
...

Issue Number: N/A

What is the new behavior?

Resolve type function is executed when using GraphQLFederationModule.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

// lib/federation/graphql-federation.factory.ts

// autoGeneratedSchema is including types with resolveType function
const autoGeneratedSchema: GraphQLSchema = await this.gqlSchemaBuilder.buildFederatedSchema(
  options.autoSchemaFile,
  options,
  this.resolversExplorerService.getAllCtors(),
);
// executableSchema "erases" this field
let executableSchema: GraphQLSchema = buildFederatedSchema({
  typeDefs: gql(printSchema(autoGeneratedSchema)),
  resolvers: this.getResolvers(options.resolvers),
});