Make the v1beta1 export lazy loaded

[inlined]

Fixes issue where loading Firestore force-loads google-gax. This adds several hundred ms latency on Cloud Functions when all we want to do is expose a native Firestore Document type from JSON.

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• [N/A] Appropriate docs were updated (if necessary)

[schmidt-sebastian]

@inlined Thanks for sending this over! This was merged as part of #197, which also removed the dependency on '@google-cloud/common-grpc`. With these two changes, loading the Firestore module now only takes tens of milliseconds instead of hundreds. Thanks!