Base POCO parsers/serializers on POCO, not IROD

[ewoutkramer]

• ⚠️ BREAKING: Based the parsers/serializers on Base instead of IReadOnlyDictionary<>. Should not really matter, since internally, the code required the things to be subclasses of Base anyway to function correctly.
• Added extensions so subtrees van be serialized, if the underlying engine is our new POCO serializer
• Added extensions so one can serialize to/from streams, if the underlying engine is our new POCO serializer.
• ⚠️ BREAKING: Removed the obsolete FhirJsonConverter constructors (use the ForFhir extension methods on JsonSerializerOptions).
• ⚠️ BREAKING: Renamed methods on the IFhirSerializationExtensions, since they made no sense (e.g. DeserializeReaderToJson has become DeserializeFromJsonReader).

Fixes #2917.