Plugin installation fails when behind proxy with NTLM authentication

It is impossible to reach out to external network when traffic is routed through proxy that requires NTLM authentication. When inspecting http.rb mentioned below I found that proxy authentication is hard-coded to Basic (on line 793), and as such, it's bound to fail on most of enterprise networks.

- Version: Logstash 5.2.0 and 5.5.1
- Operating System: Windows
- Config File (if you have sensitive info, please remove it):
- Sample Data:
- Steps to Reproduce:
1. install Logstash on a network behind proxy using NTLM authentication
2. try to install additional plugin, for example truncate
3. output is

C:\temp\elk\logstash-5.2.0>bin\logstash-plugin install logstash-filter-truncate
Looking if package named: logstash-filter-truncate exists at https://artifacts.elastic.co/downloads/logstash-plugins/logstash-filter-truncate/logstash-filter-truncate-5.2.0.zip
Net::HTTPServerException: 407 "Proxy Authentication Required"
error! at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:2641
value at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:2650
connect at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:796
do_start at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:756
start at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:745
start at C:/temp/elk/logstash-5.2.0/vendor/jruby/lib/ruby/1.9/net/http.rb:557
start at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/utils/http_client.rb:13
remote_file_exist? at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/utils/http_client.rb:23
get_installer_for at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/pack_fetch_strategy/repository.rb:27
create at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/install_strategy_factory.rb:15
each at org/jruby/RubyArray.java:1613
create at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/install_strategy_factory.rb:14
execute at C:/temp/elk/logstash-5.2.0/lib/pluginmanager/install.rb:30
run at C:/temp/elk/logstash-5.2.0/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
execute at C:/temp/elk/logstash-5.2.0/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/subcommand/execution.rb:11
run at C:/temp/elk/logstash-5.2.0/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:67
run at C:/temp/elk/logstash-5.2.0/vendor/bundle/jruby/1.9/gems/clamp-0.6.5/lib/clamp/command.rb:132
(root) at C:\temp\elk\logstash-5.2.0\lib\pluginmanager\main.rb:46

Start with lib/pluginmanager/utils/http_client.rb and the Net::HTTP connection path shown at vendor/jruby/lib/ruby/1.9/net/http.rb:793, then trace calls from lib/pluginmanager/pack_fetch_strategy/repository.rb. Reproduce plugin installation through an NTLM-authenticated proxy and verify that the truncate plugin can be reached without the 407 error.

Written by the indexing model from the issue text.