JSON: Allow weakly-typed mapping via Dictionary

[roji]

#28871 tracks weakly-typed JSON mapping via JsonDocument/JsonElement, where the JSON document schema varies and so a strongly-typed model isn't appropriate.

We can also allow simply mapping arbitrary Dictionary types - this corresponds to how many people use

• This intersects with #28688 (primitive collections in JSON).
• Both keys and values should support any valid JSON data type (string, int...).
• However we could also support Dictionary<string, object>, to map an entire hierarchy. See #26903 on this (currently seemed blocked by property bag detection).
• This should be supported at the top-level, so that the JSON object as a whole is mapped as a single dictionary (see e.g. this issue). This would mean supporting OwnsOne(x => x.Json, builder => { builder.ToJson(); }) where x.Json is a Dictionary.

[roji]

Duplicate of #29427

[ajcvickers]

Re-opening, since dictionary mapping was not covered by "primitive collections".

[vchirikov]

Link to the "issue" should be in the docs at least, was difficult to find..
Is it planned for 9.0 release?

[roji]

@vchirikov the milestone generally expresses whether an issue is planned for a given release or not.

[onionhammer]

I assume this would also meant support for ExtensionData and Dictionary<string, JsonElement> ?

[roji]

@onionhammer that would be a combination of this and #28871.

[onionhammer]

@roji putting querying aside for now, JsonElement can already be saved/serialized as an entity property, but Dictionary<string, JsonElement> cannot;