Separate Model and Interface Templates from the instantianted lists

[markcowl]

• add new property to NamespaceType 'modelTemplates' and 'interfaceTemplates'
• Move model Templates into this new property and out of the 'models' property
• Move interface Templates into this new property and out of the 'interfaces' property
• Add new optional property to ModelType/InterfaceType, 'templateType' that references the template used to instantiate the model
• derivedModels should maybe be seperated with derivedModelTemplates
• Add a flag includeTemplates in semantic walker
  • When enabled, visit the template in 'templateType' when present
  • mapChildModels should also have an includeTemplates, currently they are always excluded.
• Validate change in existing emitters and adjust as appropriate (openapi3, cadl-autorest, cadl-rpaas-controller)

[markcowl]

This follows design issue: #221

[markcowl]

David Wilson (@daviwil) what we discussed for this was determining how much effort it would take, and fitting it in, if possible.