Rework to use KUKSA.val databroker gRPC API

Rework to move from the WebSocket API with the older KUKSA.val
server to the gRPC "VAL" API of the databroker.

Changes include:
- All VISS WebSocket API code has been removed, and the signal
  providers replumbed to be driven by a new VssClient class with
  a homescreen-specific child class to hold all the gRPC API
  handling.
- The generated code for the VAL API and its dependencies has
  been checked in under lib/generated, as there still does not
  seem to be a good way to generate it during the Flutter build.
- The configuration file is now expected to be "homescreen.yaml"
  instead of "homescreen_config.yaml".  The authorization token
  field name has been renamed to "authorization", and there are
  new "use-tls" and "ca-certificate" configuration fields.  TLS
  is disabled by default for now, and the default CA certificate
  is /etc/kuksa.val/CA.pem.
- Updated .gitignore to cover a couple of generated files that
  weren't included.

Bug-AGL: SPEC-4762, SPEC-4903

Signed-off-by: Scott Murray <scott.murray@konsulko.com>
Change-Id: I1b95ed27a72435364d54ec846f2be88e3d8bb092

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