Added User-Agent String to Traffic Router Log Output

[ARMmaster17]

What does this PR (Pull Request) do?

Requested by @smalenfant, adds User-Agent header string to log output from Traffic Router for each request.

• This PR is not related to any Issue

Which Traffic Control components are affected by this PR?

• Traffic Router

What is the best way to verify this PR?

Pull and run CDN-in-a-Box and make a request against traffic router using the built-in VNC container (or your own custom solution). Check the console output of Traffic Router from the request and verify the that a field exists similar to uas="Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Firefox/68.0".

The following criteria are ALL met by this PR

• This PR includes tests OR I have explained why tests are unnecessary
• This PR includes documentation OR I have explained why documentation is unnecessary
• This PR includes an update to CHANGELOG.md OR such an update is not necessary
• This PR includes any and all required license headers
• This PR ensures that database migration sequence is correct OR this PR does not include a database migration
• This PR DOES NOT FIX A SERIOUS SECURITY VULNERABILITY (see the Apache Software Foundation's security guidelines for details)

[zrhoffman]

Just 1 comment about the changelog, everything else looks great.

[zrhoffman]

• Unit tests pass
• Integration tests pass
• External tests pass

Confirmed the user agent shows up as expected in the logs.