Add Pydantic model for new scheme

[Varbin]

The new scheme is described in #37. This WIP moves the definitions to Pydantic; it already has half of the serialization / deserialization built in.

Full list of changes:

• JSON schema:
  • Add document number and data of issue as "standard" fields
• Porting of ids to new JSON schema:
  • German temporary ID
  • German "new" ids
  • German password
  • Swiss id
• Dependencies:
  • Add Pydantic - on rare platforms (read: BSD) rust needs to be installed at compile time.
  • Fix the installation of the portals extra on Free-/OpenBSD (irrelevant to the PR, but a fix nonetheless)
  • Add tomli for reading/writing TOML documents (they may be better to read).
• Load JSON/TOML document types on startup
  • Note for implementation: Load them with importlib
• Change UI, as there is no separation between front and back with the new schema any more