A plugin hook that adds more named permissions to the list which is initially populated here:

Originally imagined this hook in this comment:

• Tool for simulating permission checks against actors #1881 (comment)

I need this for a few reasons:

• "permissions" propery in metadata for configuring arbitrary permissions #1636
  • Needs it in order to validate that permissions defined in metadata.json are set in the right place (don't set an instance permissions at table level for example)
• datasette create-token ability to create tokens with a reduced set of permissions #1855
  • Needs it to be able to register additional abbreviations for use in signed cookies
  • And for validation when you use datasette create-token and pass in extra permissions
• The https://latest.datasette.io/-/permissions debug interface needs it to add extra debug options to the <select>