Add a json field type and jsonSchema constraint support

[micimize]

Table schemas already have array and object types, but without many constraints.

Instead of adding more types/constraints (#409, #410), we could simply leverage json schemas, and thus json schema validation libraries.

Additionally, support for a generic json field that could contain any valid json string is not uncommon in relational databases (bigquery, postgres).

So, we could:

• introduce a json type
• add an optional jsonSchema constraint to json, array, and object types
• give object and array jsonSchemas their corresponding type implicitly

[nichtich]

I favour this idea but mandatory support of JSON Schema in Table Schema raises the question why to use Table Schema at all if large parts could be expressed in JSON Schema as well. Either further align JSON Schema and Table Schema (e.g. use same regex syntax instead of two different dialects) or make JSON Schema constraints optional.

[nichtich]

There is no need for an additonal json type as array and object are already defined as valid JSON. Support of JSON Schema could be implemented by an additional jsonSchema constraint. JSON Schema could also be allowed to constraint strings and numbers but this would cause confusion which schema language to use, so I'd restrict it to objects and arrays.

[micimize]

@nichtich I no longer work with frictionless so no longer have the context in my head or a stake – will unsubscribe and leave this open as it still seems in-use for tracking.