Allow packages to supply a dynamic config schema

[j005u]

This change allows packages to supply an executable at /opt/etc/package-config/package-name/schemaV2 and /opt/etc/package-config/package-name/uiSchemaV2 that outputs the appropriate schema instead of a static .json file.

For example, msp-osd can populate font options dynamically.

The --config-schema and --config-ui-schema flags get appended accordingly to executable calls to allow for easy symlinking to executable that might serve multiple purposes.

[j005u]

Here's an example schemaV2 executable for msp-osd schemaV2.zip