JSON schema policy operator for JSON object metadata parameter

[Razumain]

In the workshop in Stockholm we discussed this policy operator that gained a majority of the votes supporting inclusion.

This policy operator allows expression of an array of JSON schemas as the policy operator value.

The operator value match the metadata parameter if, and only if:

• The metadata parameter is a JSON object
• The metadata parameter value matches all present JSON schemas in the policy operator.

This policy operator is merged to the union of all JSON schemas of the merged policy operators.

Rationale:
Some metadata structures (in particular in the Wallet environment) use JSON objects as metadata parameters. These JSON objects can have a quite complex structure. A JSON schema offers a comprehensive but standardized way to express requirements on the content. Verification of the metadata parameter value against a schema is easy with publicly available JSON libraries. This makes support of this policy operator very easy to implement.

[selfissued]

This is a quite large additional ask for implementations and many won't need it, since it requires an implementation of JSON schema. Policy operators are an extension point. Therefore, this seems like a perfect candidate to write up in its own spec, after which we can gain implementation experience among those who want/need this. Adding this as an extension would be a non-breaking change.