Feature: Add User Agent Information

[jvanderaa]

It would be good to enhance things further to provide the option to have a header that gives the user/agent.

[joewesch]

There was already a PR to add this and a conclusion that it is already possible: #171 (comment)

[nrnvgh]

My two cents: We actually ran into a case recently where having the client send a vendor-side user-agent would have been useful; specifically, being able to run reports to determine what version of pynautobot our customers were using; having that in the user-agent header would have made things easier.

[joewesch]

Closed via #207