ProtoBuf/gRPC support

Problem statement

Because health connect data (especially in Full mode) can contain many data points, this often creates huge JSON blobs that are inefficient to transfer over the network and can run into Content-Size limits, especially if doing an initial sync of a longer time period.

Proposed solution

Implement support for posting data over gRPC, with a protobuf schema. This allows data to be sent in a more efficient format. Protobuf/gRPC is particulary good for this application because it's a generic format that can be used in many programming languages, by automatically generating a server implementation from the published schema (.proto) files.

Alternatives considered

Other binary encodings can be used, or perhaps implementing sending larger JSON blobs in multiple "pages" instead of all at once.

Impact

Users wishing to backfill past data into an external service, applications that demand high efficiency in data transmissions.