Cosmos: Support for weakly typed json

[IGx89]

Currently there's no good way in EF Core to work with fields in Cosmos that can contain arbitrary JSON. We would love a way to both read and write to fields like that.

The Npgsql provider appears to have elegantly solved this by adding support for JsonDocument column types (ref: http://www.roji.org/efcore-pg-advanced-json#weakly-typed-access-via-jsondocument). Having matching functionality with the Cosmos provider would be exactly what we need.

[ajcvickers]

@AndriySvyryd I can't find the dupe for this.

[IGx89]

I did a lot of searching for dups myself before creating this :). I found similar issues, but all were vague or had replies of the sort saying this would be great but we need to wait until the other providers solve this first. Now that another provider has taken the initiative on picking a solution, hopefully it's a good time to go forward with this provider as well!

[AndriySvyryd]

Most of the functionality would be covered by #17306 and #17311, albeit the API wouldn't be as nice

[IGx89]

Thanks for considering this! FromSql would work OK for reads but not well/at all for updates and creates, and a custom serializer wouldn't work for our specific use case (we already tried it) due to our use of dynamic JSON (we have a JSON object with string keys and arbitrary object values -- basically Dictionary<string, JObject>)

[IGx89]

I assume this is dependent on #18753, since JsonDocument is from STJ which Microsoft.Azure.Cosmos switches to in v4.

[ChiefInnovator]

Agreed! This would be a great feature.

[erikrenaud]

It woud be nice if we could use HasConversion to return pure json and viceversa.
I might be able to use this the issues i have because of #9630.

[ajcvickers]

Note from triage: this needs to be done in conjunction with or after #17306.