Add endpoints for plans and plan_data

To mirror the functionality for tasks, we want to be able to list plans and return information about them from Puppetserver. This entails a few steps:

• Extend the Puppet::Server::Master ruby code to query the info_service for plans (https://github.com/puppetlabs/puppetserver/blob/master/src/ruby/puppetserver-lib/puppet/server/master.rb#L113-L129)
• Extend the JrubyPuppet interface to expose those methods to clj (https://github.com/puppetlabs/puppetserver/blob/master/src/java/com/puppetlabs/puppetserver/JRubyPuppet.java)
• Extend the JrubyPuppetService protocol to expose the new functionality https://github.com/puppetlabs/puppetserver/blob/master/src/clj/puppetlabs/services/protocols/jruby_puppet.clj
• Extend the jruby-puppet-pooled-service to implement the new functions in the protocol https://github.com/puppetlabs/puppetserver/blob/2deb686496862e30272b9cf05f07f7419cbbfe7a/src/clj/puppetlabs/services/jruby/jruby_puppet_service.clj
• Add the appropriate handlers and middleware functions to expose the HTTP endpoint - https://github.com/puppetlabs/puppetserver/blob/e30ae3295214ddb11156cb5fa62f3f8b8978e060/src/clj/puppetlabs/services/master/master_core.clj#L812 is the top of this, and goes through couple of layers before hitting the TK service

These new endpoints should match those for tasks, within reason.