generate package:googleapis_firestore_v1 with http routing headers

[devoncarew]

• generate package:googleapis_firestore_v1 with http routing headers

This is a partial proof-of-concept of supporting http routing headers; for more context, see google/protobuf.dart#1018.

This generated output creates correct headers for unary calls, but doesn't generate correct code for streaming calls. For that, we'll need to augment package:grpc so that generated clients can inspect the first request on the stream and set http headers from that information.

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