Add `userAgent` JDBC param to Avatica

It could be beneficial to add a userAgent parameter to the JDBC URL recognized options. This userAgent value would be passed from the client to server as a User-Agent HTTP header overriding Avatica's current default of Apache-HttpClient/<version> (<java_version>). This would allow Avatica servers to more easily identify requesting applications.

Here's Denodo's userAgent as an example.