Standardise `Argus` access policy signals in metadata

# Background

There are already a variety of metadata fields that should impact of `Argus`, such as

- https://github.com/Joystream/joystream/issues/4326
- https://github.com/Joystream/joystream/issues/4325
- https://github.com/Joystream/joystream/issues/4327
- https://github.com/Joystream/joystream/issues/4331

# Question

1. These policy implications are tied to very specific metaprotocol messages, but this means that if an application developer wants to augment these, even in very superficial ways, `Argus` will not automatically understand these policy signals in these new message formats. **Seems the answer here has to partially be to lift out policy stuff into a special layer, sort of like what has been suggested for application actions.**: #4307
2. How can we allow application developers to define _new policies_ entirely? e.g. only x users can access something, or only people with an NFT e.g. It would be very powerful if there was some process to standardize such things. **Addendum: this may just not be possible because the DAO is paying these costs, a totally unconstrained way of imposing such obligations does not make sense. The next best alternative may just be to have a really broad suite of policies, and then new ones have to be adopted through governance, here are some interesting initial ones
- is among this set of members.
- is among people holding an NFT worth at least x for at least y time.
- is holders of at least x balance in $CRT

Its not clear if gateway still should pay in all cases.