As a user I can filter, via a JQL Function, all Tests associated with a Test Plan board folder

As a user, I should be able to use a JQL function "TestPlanFolderTests(testplanKey,folder,[descendants],[status],[environment])" to obtain a list of Tests within a given folder path of the Test Plan board, optionally that are in a given status, for a given Test Environment. 

Optionaly it should also should be possible to return all descendants (false by default) or just the direct child ones.

Arguments shall behave consistently with Xray conventions:

• status can be a single value or multiple values separated by pipes (|)
• environment can be left empty (as '' or "") to represent All environments or the value "__NULL__" to represent empty environment