Add threat model proposal

[rficcaglia]

Adding the proposed Threat Model OSCAL schema. This PR includes:

• an overview README on the proposal itself
• supporting README on STIX 2.1 based concepts that are referenced in the main README, but are not required to the core schema per se

@ancatri @degenaro @vikas-agarwal76

[degenaro]

I created a repo here: https://github.com/oscal-compass-lab/prototype-threat-model