GRPC proto cleanup

[argerus]

1. Move metadata fields to .metadata in Datapoint.

   This makes it more clear what VIEW_METADATA refers to and it makes it possible to have FIELD_METADATA.

2. Add fields in addition to view in Get & Subscribe

   This makes it possible to subscribe to single fields. It's not really needed for Get but it seems consistent to add it to both.

   Because of this I also removed for example VIEW_UNIT since it's now equally easy to add FIELD_METADATA_UNIT to fields. In general I think view is a good solution to having a default view and a convenient way to request multiple fields. But perhaps not for specifying individual fields.

3. Include which fields have changed in subscription replies.

   In order for a client to tell which field has changed (e.g. if it subscribes to both value and actuator_target).

4. Various cleanup

   • Fix typos, prefix all enums, update documentation, removed references to Status
   • Add [field: FIELD_NAME] to all fields to make it super obvious which is which.

Not included

• I still think there should be some way to specify that a value is not available. Compare with Android VHAL, that has VehiclePropertyStatus for every VehiclePropertyValue which can be AVAILABLE NOT AVAILABLE or ERROR. And databroker which has the same concept embedded in the oneof value. The point is that a missing value isn't even necessarily an error and it should still be possible to at least Get and Subscribe to such a Datapoint.

[SebastianSchildt]

Argh, this was supposed to be already in. Must have messed sth up with git...