feat: Add Zoo (DO NOT MERGE)

[SilviaZeta]

Description

• add the new fields description, is_public and origin to the Plugin model (and migrations!); origin is a ForeignKey that is set to None for origin plugins and it is a link to the origin plugin for copies of a plugin.
• add the new fields public_key and encrypted_access_key to the TrustedService model; for these fields None values are allowed to facilitate the migration from the old to the new system, due to the fact that values for these fields must be generated by DOMINATE.
• refactor plugins, so as to resolve the redundancy in plugin.py and trusted_service.py.

relates to gliff-ai/roadmap#82

DO NOT MERGE!

Checklist:

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