[Fleet Console] Add skeleton for API to return Device Manager data

This CL includes:
- Adds a new proto for defining a new DM-like API (currently a subset of DM's protos but expected to evolve over time)
- Sets up a DM-like pRPC API which only returns empty responses.
- Also includes a rough skeleton of client code directory setup.
- Added a client proto code generation script

Notes:
- Future CLs will populate these endpoints with data from Device Manager.
- Protos copied from https://source.chromium.org/chromiumos/chromiumos/codesearch/+/main:src/config/proto/chromiumos/test/api/device_manager_service.proto but pruned to only contain methods that will be used by the new UI.
- Over time, this prototype's protos are expected to diverge from DM as we add additional data sources such as UFS.

Bug:356414150
Change-Id: I179e092f54301f0aaecf51e82494f87c5886e320
Reviewed-on: https://chromium-review.googlesource.com/c/infra/experimental/+/5833917
Reviewed-by: Justin Suen <justinsuen@google.com>
12 files changed