Adds an abstraction layer to switch between serialization formats.
Currently only supports JSON. The abstraction layer supports
`String`, `Int`, `Array`, `Null`, `Boolean`, `Float` and `Object`. This
feature is only CPP complaint.
To write from a stream, the structure can be built by creating a value
(any subclass of `blender::io::serialize::Value` can do, and pass it to
the `serialize` method of a `blender::io::serialize::Formatter`. The
formatter is abstract and there is one implementation for JSON
(`JsonFormatter`).
To read from a stream use the `deserialize` method of the formatter.
{D12693} uses this abstraction layer to read/write asset indexes.
Reviewed By: Severin, sybren
Maniphest Tasks: T91430
Differential Revision: https://developer.blender.org/D12544
