Proposal: Use a specific/custom user-agent for DAST

Inspired by #366550, this feature proposal seeks to make troubleshooting DAST easier:

• What if we permitted people to control the user-agent? (What if the user-agent was a combo of job ID and namespace info?)

Building on that feature proposal, perhaps the user-agent should be something static that GitLab decides that can optionally include a job ID, example:

gitlab-dast vx.y.z

gitlab-dast vx.y.z JobID:01234567

This feature should not be necessary in the vast majority of DAST cases. However, during DAST troubleshooting this additional information can be helpful to customers and Support Engineers.

It is not necessary for the user-agent to be a user-supplied value or for it to be a dynamic value. However, the current user-agent is not easily distinguished from other Web traffic. (We understand that users can alter this and that user-agent is not a guarantee.)