JSON: support for weakly typed json properties

[maumar]

Currently we only support strongly typed POCO objects that map to JSON and are reflected in our metadata. We should also allow weakly typed properties, i.e. JsonDocument/JsonNode, that would not be reflected in our metadata and support query via GetProperty and various Get* methods (e.g. GetInt32) to retrieve the final scalar result.

[AndriySvyryd]

The Cosmos issue for this - #20584
Depends on #30604

We should consider exposing it through Dictionary<string, object> or any type with a compatible indexer property instead of depending on a specific DOM implementation.

[Ciantic]

Postgresql's EF Core allows direct mapping with JsonDocument, e.g.

public class SomeEntity : IDisposable
{
    public JsonDocument Customer { get; set; }
}

JsonDocument is automatically recognized and mapped to jsonb

More details here: https://www.npgsql.org/efcore/mapping/json.html#jsondocument-dom-mapping

I would prefer these to be consistent, right now I'm writing a library that I would like to work with Postgresql and SQLite, I have to write HasConversion() with JsonDocument.WriteTo and Parse for the SQLite, if EF Core supported JsonDocument directly things would be more consistent.

JsonDocument has one drawback it is IDisposable, it makes all entities IDisposable as well, this is not what I would want though.

I think JsonNode could be a better value type.