Customizable HTTP user agent for oc for mirroring analysis on Quay.io

1. Proposed title of this feature request

Customizable HTTP user agent for oc

2. What is the nature and description of the request?

oc should provide the ability to run with a customer HTTP user-agent string. The setting should be implemented via a CLI switch and/or environment variable. This setting should also be configurable when vendoring in the actual image mirror code into other oc subcommands (e.g. oc adm release mirror and oc adm catalog mirror). An alternative would be to use an distinct user-agent name in oc altogether.

3. Why does the customer need this? (List the business requirements here)

A lot of traffic is generated by oc when mirroring OCP release payload or OperatorHub content. To segregate this kind of traffic from regular client traffic we need to have some sort of differentation property. User-agent seems to be a viable choice.

4. List any affected packages or components.

oc