Support inheritence with JSON POCO mapping

[roji]

System.Text.Json has polymorphic deserialization in 7.0. We should implement a similar scheme here with $type, so that JSON documents we produce are compatible with that.

Originally requested in npgsql/efcore.pg#2321

[maumar]

related: #9630

[atrauzzi]

Copied from my original issue

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Support in EF7 and upcoming work in EF8 for JSON is coming along nicely, and I was wondering if there might be room to continue expanding on it.

This specific idea is to support of interfaces or otherwise abstract types in JSON columns, through the use of a type hint that gets embedded with the data.

I've accomplished this in the past by writing my own .HasConversion, accompanied by some custom System.Text.Json [de]serialization voodoo. This ended up being quite useful for working with the data after it was retrieved, but obviously didn't fully integrate with EF in terms of being able to filter on properties defined by the contracts.

An example of how this could be useful is for storing arrays/collections as part of a known JSON type:

{
    "configurations": [
        {
            "_discriminator": "configuration-type-1",
            "specificToAll": "All types would have this.",
            "specificToOne": "Only type one would have this."
        },
        {
            "_discriminator": "configuration-type-2",
            "specificToAll": "All types would have this.",
            "specificToTwo": "Only type two would have this."
        }
    ],
    "singleConfiguration": {
        "_discriminator": "configuration-type-3",
        "specificToAll": "All types would have this.",
        "specificToThree": "Only type three would have this."
    }
}

This could be deserialized into something like:

public class ApplicationSettings
{
    public IList<Configuration> Configurations { get; set; } = new();
    public Configuration SingleConfiguration { get; set; }
}

public interface Configuration
{
    public string SpecificToAll { get; }
}

public class ConfigurationTypeOne : Configuration
{
    public string SpecificToAll { get; set; }

    public string SpecificToOne { get; set; }
}

public class ConfigurationTypeTwo : Configuration
{
    public string SpecificToAll { get; set; }

    public string SpecificToTwo { get; set; }
}

public class ConfigurationTypeThree : Configuration
{
    public string SpecificToAll { get; set; }

    public string SpecificToThree { get; set; }
}

New EF-based JSON filtering would only allow filtering on the fields it has reason to expect, so in this case, SpecificToAll. But that alone would be quite powerful as this technique allows for a lot of dynamism in the schema without requiring migrations.

[marchy]

This would be extremely useful.

Currently anything other than simple "complex" structures aren't supported by EF Owned Types (either multi-column-mapped or JSON-column-mapped), limiting their use and having to fall back to string columns and managing our own JSON.