Feat: support supplementary sign from osc

This PR supports parsing supplementary signs from osc files.

In order to support this (example template):

1. A supplementary sign defined in OSC file should contains a special property mounted_sign referring to the name of its main sign.
2. The order of defining main signs and supplementary signs in an OSC file does not matter.

Additional Reference: Integration tests within gt-gen-simulator using this new feature have also been conducted locally (for reference).