Improve BP_Signup::get() function whitelisting the orderby argument

[imath]

Working on the BP REST API documentation made me see a task hasn't been completed since BP 2.0.0 : whitelisting the orderby argument. My bad :(.

It's about time to do it so that the BP REST API Signup endpoint can have improved collection parameters.

[imath]

In 12635:

BP Signup: add a whitelist of available orderby arguments

The BP Signup::get() method orderby argument's possible values are : signup_id, login, email, registered and activated. The Default value remains signup_id.

Fixes #8284