Added tables for `activitypub`

[aileen]

ref MOM-122

• Added prototype table for activitypub and actors_keys
• The table is classed as prototype, as we're not expecting this to be the final version
• Added json as valid type for schema test

[github-actions[bot]]

It looks like this PR contains a migration 👀 Here's the checklist for reviewing migrations:

General requirements

• [x] Satisfies idempotency requirement (both up() and down())
• [x] Does not reference models
• [x] Filename is in the correct format (and correctly ordered)
• [x] Targets the next minor version
• [x] All code paths have appropriate log messages
• [x] Uses the correct utils
• [x] Contains a minimal changeset
• [x] Does not mix DDL/DML operations

Schema changes

• [x] Both schema change and related migration have been implemented
• [ ] For index changes: has been performance tested for large tables
• [x] For new tables/columns: fields use the appropriate predefined field lengths
• [x] For new tables/columns: field names follow the appropriate conventions
• [ ] Does not drop a non-alpha table outside of a major version

Data changes

• [ ] Mass updates/inserts are batched appropriately
• [x] Does not loop over large tables/datasets
• [ ] Defends against missing or invalid data
• [ ] For settings updates: follows the appropriate guidelines