Request cbauth accept HTTP User-Agent

Query has MB-51833 which requires we set the HTTP User-Agent header to identify requests coming from Query in the http_access.log.   Query of course uses cbauth so doesn't control the HTTP requests directly.

This improvement request is for cbauth to:

• Make use of the User-Agent in the request passed to AuthWebCreds()
  • I expect means just copying such a header in VerifyOnServer()
• Provide a means (perhaps a new function?) of supplying a user agent string for use with Auth()
  • say an AuthWithUA() or the like function accepting a third string argument as the User-Agent string which Query can change to use.