Consider separating service protos into different files

[ahmetb]

Currently all files are in a single proto. https://github.com/GoogleCloudPlatform/microservices-demo/blob/4572974c882b7c159a59e5344a3aa42c7f4af040/pb/demo.proto

Ideally we should have per-service protos like pb/hipstershop/currencyservice.proto etc.

[daniel-sanche]

Closing this and other kind/improvement issues as we attempt to move the project into a stable maintenance state

[askmeegs]

Reopening to resume the discussion and triage.

[mathieu-benoit]

Good candidate to be tackle in https://github.com/GoogleCloudPlatform/microservices-demo/issues/814 too.

[NimJay]

Commenting to cool this issue down (from our team's out-of-SLO list): We should still address this issue. I'll leave it open.