Add ability to duplicate an agent and a flow

Problem

In order to use GitLab created and Community created agents/flows with our own customizations, we need the ability to duplicate them.

Proposal

Add a button Copy / Fork / Use as Template to agents (or flows). When clicked, it should open the existing create/new form with fields pre-populated from the original.

• The project ID will not be pre-filled, so the user must choose the target project.
• The visibility level will reset to the default (with the option for the user to set it explicitly).