Add a simple ActionCache CLI client

Context

Being able to query the ActionCache content manually is something frequent that one does when debugging a build. BuildGrid should come with a simple bgd action-cache CLI tool wrapping GetActionResult() and UpdateActionResult() calls.

Task Description

• Introduce a bgd action-cache CLI application with:
  • bgd action-cache get ACTION_DIGEST_STRING
  • bgd action-cache update ACTION_DIGEST_STRING RESULT_DIGEST_STRING

Acceptance Criteria

bgd action-cache allows query and update BuildGrid's ActionCache service.