[cml] Add rust cm_json library

This change moves some cmc code into a dedicated cm_json library,
which provides common Rust facilities for handling component manifest
files in JSON format.

Any functionality added to this library should be usable from host
tools.

Some of this functionality will be used by the upcoming CM->FIDL
translator.

TESTED=added cm_base_tests, although currently empty

Change-Id: I41665ea2c5c271d09614dfd8cfcf6162cc16581f
14 files changed