feat: define new roles for Castopod instance and podcasts

Is your feature request related to a problem? Please describe

Currently, there only exists 2 roles in Castopod:

• superadmin (the highest and only role that can do anything)
• podcast_admin (the highest role to manage everything pertaining to a certain podcast)

Even though you may create new roles directly in the database, it is quite tedious. And by default, the choice of giving the ability to do everything or nothing to users is not a good experience.

Describe the solution you'd like

⚠️ Before adding roles, replace myth-auth with shield. Shield is an official library, more up to date and flexible alternative to authentication. It would benefit greatly for future developments.

Add new default roles that come with any Castopod instance: guest, editor, ...

Describe alternatives you've considered

Add the ability to create new roles directly from Castopod (out of scope for now).

Additional context

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