* Incremental work on context activity
* Add extensions to get and set context params
* Update polymorphic converter to handle objects serialized to primitives
* Add convenience factory methods for SetWorkflowContextParameter activity
* Add API endpoint to list available provider types
* Implement workflow context provider selection property panel
* Update context provider picker to be based on selected providers
* Make SendEmail support different types of input for To, Cc and Bcc fields
* Fix EF Core WorkflowDefinitionStore to use polymorphic serialization for metadata
* Add JS support for getting workflow contexts
* Add Installed Features endpoint
* Update polymorphic object converter to skip start array
* Update activity execution middleware to handle background execution
* Fix sample program to include activity middleware for contexts
* Comments and cleanup
