Add the ability to identify key elements in the ModelInfo

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

Start with the ModelInfo and ModelInfoSettings generation paths, then read the linked FHIR Implementation Guide definition of Key Elements. Trace how model information currently comes from configuration and determine the affected StructureDefinition extensions and per-type values. Done means ModelInfo represents Key Elements and the listed model and type settings are derived from StructureDefinitions.