3049 Use poco based parsers

[ewoutkramer]

FIxes #3049.

This PR:

• Moved the code from FhirXml/JsonPocoDeserializer to the existing original and old FhirXmlParser/FhirJsonParser.
• It reuses ParserSettings, but adds the newer settings necessary for the newer POCO deserialization code
• It re-instantes the old methods from FhirXmlParser as extension methods on the FhirXmlParser, so that it has both the newer POCO "deserialize" methods + the existing "Parse" methods.

The idea is that old code keeps compiling, but will start using the newer POCO code.

The biggest change is that the newer parsers are strict by default, so some unit tests (and FhirClient and DirectorySource) had to be changes to switch back to RECOVERABLE/OSTRICH.

[ewoutkramer]

Reviewed it for snafus, added come helpful comments for reviewer (I hope). No formal PR review.

[Kasdejong]

approved. One comment: will this not hugely interfere with andrzejs PR?

[ewoutkramer]

[Merge branch 'develop-6.0' into 3049-use-poco-based-parsers](/FirelyTeam/firely-net-sdk/pull/3073/commits/aed8ef59b99f1e00437ec87a4fe631745d182788)

Verified
[aed8ef5](/FirelyTeam/firely-net-sdk/pull/3073/commits/aed8ef59b99f1e00437ec87a4fe631745d182788)

Yes, it will. Maybe I should not pull this. It seems it's easier to integrate his changes into my PR than my changes into his PR.

[ewoutkramer]

Actually, no, because I did the renames in a separate project! So I am pulling this and will update his PR with develop6 afterwards.