GraphQL Federation - typedefs merged with typePaths

[ludorenzetti]

PR Checklist

Extends GraphQL Federation module to use the typeDefs in addition to typePaths. The feature is already available in NestJS GraphQL base module.

• 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?

[ ] Bugfix
[x] 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?

const myTypeDefs = gql`
 
extend type Query {
    ...
}

`

GraphQLFederationModule.forRoot({
    typePaths: ['./**/*.graphql'],
    typeDefs: [myTypeDefs] // Override by typePaths
})

What is the new behavior?

const myTypeDefs = gql`
 
extend type Query {
    ...
}

`

GraphQLFederationModule.forRoot({
    typePaths: ['./**/*.graphql'],
    typeDefs: [myTypeDefs] // Merged with typePaths
})

Does this PR introduce a breaking change?

[ ] Yes
[x ] No

Other information

[kamilmysliwiec]

Thanks!