As a user, I can define Test Step custom fields at project level

As a user, I can define Test Step "custom fields" at project level, so that users can define their values during the specification (in the Test issue screen; not on the execution screen) on each Test case. 

ACs: 
 * it should be possilbe to create, in the project settings, Test Step custom field types of string, number, date, boolean, multiselect, singleselect 
 * it should be possilbe to define a Test Step custom field as mandatory to be filled or not 
 * it should be possible to define default values for the Test Step custom fields