Consider exposing a serialization mechanism that supports open hierarchies for `AIContent`

[eiriktsarpalis]

The AIContent type uses JsonDerivedTypeAttribute to support polymorphic serialization which by design is constrained to the types defined within the M.E.AI.Abstractions assembly only. We should consider exposing a mechanism that lets users register converters for custom content types, in a way that is compatible with the source generator and Native AOT.