Separate Model and Interface Templates from the instantianted lists

- [ ] 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)