Add support for generating Open Threat Model (OTM)

[stevespringett]

Hello. I'd like to be able to generate OTM from Threagile. For example, adding an option for --generate-otm would be ideal.

The Open Threat Model format is still early in development, but its goals are to standardize how data from threat models are represented, providing interoperability between different systems and tools.

Per the readme:

OTM allows both humans and computers to understand what are the components of a system, how are they distributed, the security risks that could be exposed to attackers and the mitigations that could be implemented to avoid those vulnerabilities.

OTM can be used to document your system and threat model, to keep you threat model aware of the changes that happens in the system and many other use cases.