Mapping template planner: execute against real data sources. v1 #3

As a user, I want to be able to execute the resulting evaluated mapping template against a real data source.

I should be able to select existing data sources in the currently selected API. When the mapping template is evaluated with the given context object, the result should be "executed" against the selected data source, and the result returned.

Note: request Mapping Templates only (e.g. DynamoDB GetItem)

A warning (info) should be shown to the user that it is just a simulation and that the result is interpreted "locally", not on the real API