feat(ApolloFederation): Add "_service" resolver

[minhaj-shakeel]

Fixes GRAPHQL-934.
This PR adds _service resolver according to the federation specs which return generated schema string without @apollo directives to be used by the gateway.
The query is:

query {
   _service {
      sdl
   }
}

which returns the schema string.

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This change is 

[abhimanyusinghgaur]

Reviewed 3 of 5 files at r1, 2 of 3 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @pawanrawal)