NIFI-5282: GCPProcessor with HTTP Proxy with Authentication

[ijokarumawak]

NOTE for reviewer

This PR contains two commits. The 1st one is submitted by @jugi92 via #2943 . I have reviewed the PR and updated few points. Please refer #2943 comments I posted for where I have changed. With those changes, I'd give +1 to #2943, so the 1st comment has 'this closes' comment.

The 2nd commit includes refactoring mentioned above, and also adds ProxyConfiguration support on top of the 1st commit.

Please DO NOT squash these two commits when merging, in order to keep @jugi92 contribution. Thanks!

Some Docker commands to run Squid proxy server

# Start a Squid container
docker run -d -p 3128:3128 --name squid salrashid123/squidproxy tail -f /dev/null
# Login to the container
docker exec -it squid bash
# Start proxy without auth
squid/sbin/squid -s -f squid.conf.forward
# or with basic auth
squid/sbin/squid -s -f squid.conf.basicauth
# Stop
squid/sbin/squid -k shutdown

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[mcgilman]

Reviewing...

[mcgilman]

Thanks @ijokarumawak! This has been merged to master.