Support serializing json fields as strings

[arves100]

During analysis in brave frontier (for example:

packet-generator/assets/mst/slots.kdl

Line 151 in 37256a2

json VideoAdsSlotGameInfo {

), it was shown that Brave Frontier serializes nested JSONs formatted as string

The generator must support the following new syntax:
JsonType::str as serializing the JSON as a string.

A new glaze mapping type would be required in order to do so (for example: pkg::glaze::json_as_string<Type>)

The default behavior of JSON nodes should be to serialize them as JSON objects.

This will allow us to configure stuff like the Video ad slot MST.

[andry-dev]

This is supported in the parser already, just use type="Foo::str" with Foo a JSON definition. See the following in the IR:

packet-generator/src/intermediate/mod.rs

Lines 340 to 343 in 37256a2

	Definition {
	encoding: JsonEncoding,
	definition: DefinitionRef,
	},

However, the C++ / Glaze generator currently ignores it.