Add additional context to Inquiries

A user in Slack mentioned he would like to be able to store additional useful information that can help inform a response to an inquiry. The use case is that core.ask is used before applying a terraform plan - it would be nice if the user could get prompted with the output from terraform plan before they have to respond to the inquiry. Right now the only way to do this is to place the output into the datastore and then look it up in a notification workflow that responds to an inquiry trigger.

I still firmly believe that the ID and route are the only fields that should be in the inquiry trigger. It's more RBAC-friendly. But we could consider adding a field to the inquirer runner for storing useful context like this. Users can see this field after retrieving the full inquiry via the API, as they have to do today to see other fields like the schema.

Should also add an st2.get_inquiry or similar action, to make it easier to retrieve inquiry details.

Start by reading the existing core.ask inquiry flow and how full inquiry details are retrieved through the API. Define what context should be stored and exposed, then determine the scope of a possible st2.get_inquiry action; done means the requested inquiry context and retrieval path are available without changing the inquiry trigger fields.