Decide whether to expose advanced extensions for IFhirSerializationEngine

[ewoutkramer]

PR #2950 (and #2913 before it) introduced a few extension methods to "reach out" under the limited IFhirSerializationEngine interface to be able to serialize from/to streams and serialize subtrees.

One could argue that IFSerEng was never made for that, and so one should use the lower-level serializer API. However, it is not yet easy to construct such parsers according to our rules for Ostrich/Backwards compat etc, since that is introduced in the higher-level API. So, ideally, one should be able to initialize the BaseXml/JsonPocoSerializers with the same setup as the Engine, so one can then call the lower-level API.

In this case, we can remove these extensions.

[ewoutkramer]

We are going to remove the whole SerializationConcept in favor of working with the serializers/parsers directly when we revisit the parsers, so this is no longer a real issue.