Implement InstantiateTemplateRequest

[dbdness]

Is your feature request related to a problem? Please describe.
The InstantiateTemplateRequest SDK request is currently not supported in XrmMockup - plugins executing this request fail with the following error under unit test:

System.NotImplementedException: Execute for the request 'InstantiateTemplate' has not been implemented yet.
This exception was originally thrown at this call stack:
DG.Tools.XrmMockup.Core.ExecuteRequest(Microsoft.Xrm.Sdk.OrganizationRequest, Microsoft.Xrm.Sdk.EntityReference, DG.Tools.XrmMockup.PluginContext)
DG.Tools.XrmMockup.Core.Execute(Microsoft.Xrm.Sdk.OrganizationRequest, Microsoft.Xrm.Sdk.EntityReference, DG.Tools.XrmMockup.PluginContext)
DG.Tools.XrmMockup.MockupService.SendRequest(Microsoft.Xrm.Sdk.OrganizationRequest)

Describe the solution you'd like
Support of the InstantiateTemplateRequest as documented here.