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

As a user, I can define Test Run "custom fields" at project level, so that users can define their values during the execution of each individual Test Run.

For this, it should be possible to:

• provide some additional fields in the execution screen of a Test Run and in the REST API (Xray JSON endpoint and Test Run related endpoints)
  • these fields should be dynamic
• make new reports based on these fields (e.g. evolution of "CPU usage" through time)
• update existing reports (e.g. Test Runs Report) to show these fields

It should be possible to create, in the project settings, Test Run custom field types of string, number, decimal, date, boolean, multiselect, singleselect

It should be possible to define a Test Run custom field as mandatory or not