Apply OTM schema to OTM files

[Jon Gadsden]

Describe what problem your feature request solves:
At present when Threat Dragon reads an Open Threat Model file, it does not check it against the OTM schema at https://github.com/iriusrisk/OpenThreatModel/blob/main/otm_schema.json

Describe the solution you'd like:
Threat Dragon should check the OTM file against the schema

Additional context: