Add utility functions to load policies/schemas from files

[charithe]

Add utility functions to enable users to read policies or schemas from YAML or JSON files/strings and load them using the Admin API.

[haines]

I wonder if implementing this client-side might cause headaches with different languages' YAML handling. What if we added RPCs so that we can send the raw file contents and let the server handle the parsing?

[charithe]

The other SDKs use the protobuf JSON handling functions for this. I'd expect them to be consistent across languages (at least for the languages Google officially supports).

Not opposed to adding a new RPC to handle raw files but I feel like we'd make the API complicated by having two ways to do things.