metrics: Implement loading metric extension directory

You can now pass in --metric-extension flag to trace_processor_shell.
The flag is in the format disk_path:virtual_path.

Any internal metric proto/sql files matching the virtual_path location
is skipped. All protos in the disk_path are loaded with
"protos/perfetto/metrics/virtual_path/" location, and all SQL files are
loaded in the "virtual_path/" location.

Design doc: go/perfetto-runtime-metrics-import

Bug: 182165266
Change-Id: I6ce3d5881219e5099b6eb9c6f653b1dfded51297