Add the ability to identify key elements in the ModelInfo

[brynrhodes]

In addition to "mustSupport", FHIR Implementation Guides now include the concept of "Key Elements", which are defined here:

https://build.fhir.org/ig/FHIR/ig-guidance/readingIgs.html#sdviews

We should add to the ModelInfo a reflection of whether an element is a "Key Element"

We should also change the ModelInfo generation so that it drives off of StructureDefinition extensions for model info information, rather than pulling from a configuration:

ModelInfoSettings:

• ModelName (e.g. QICore)
• ModelUrl (e.g. http://hl7.org/fhir/us/qicore)
• PatientClassName (e.g. Patient)
• PatientBirthDatePropertyName (e.g. birthDate)
• TargetQualifier (e.g. qicore)
• TargetUrl (e.g. http://hl7.org/fhir)

And for each type, we need to be able to set:

• Retrievable (defaults to true for Classes, false for Types)
• Label (defaults to StructureDefinition.Title without Model Prefix if present)
• PrimaryCodePath

[c-schuler]

Related #217