As a user, I can enable/disable Tests management under Test Plans whenever using Structure

As a user, I can enable/disable Tests management under Test Plans whenever using Structure, so that I can see/manage the Tests association with Test Plans directly.

A flag shall be provided in Xray extender (checked by default): "Manage Tests under Test Plans".

Note: the current, as of v3.1, behaviour has this management always enabled by default, as a consequence of XRAY-1173.