Add support for setting a user-agent in preferences for discovery

[n8felton]

Is your feature request related to a problem? Please describe.
While attempting to connect to a vendor URL, I get 403 Forbidden when I do not provide a user-agent with the request headers.

Describe the solution you'd like
Possible suggestions:

1. Use the current system's Safari user-agent. I'm not sure if this can be queried from the system anywhere, or if it would need to be generated
2. The ability to set a specific user-agent in the preferences.
3. The ability to set a specific user-agent with a command-line argument.

Describe alternatives you've considered
Manually creating the recipe by hand.

Additional context
To see what I mean:

curl -LI http://upgrade01.sky.shoretel.com/ClientInstall

vs

curl -LI -A 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/75.0.3770.100 Safari/537.36' http://upgrade01.sky.shoretel.com/ClientInstall