Move HttpSupport's configureHttpProxy to jgit-pgm

This is the last chunk of code in jgit-core which references the awtui
package.  Moving it to the only consumer in jgit-pgm allows us to move
the awtui package over to the jgit-awtui module.

Change-Id: I2fd81be2076117b2f2c5f8ed45de7f29272af6cf
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>