Deprecate path.plugins in 5.x and remove in 6.0

Background: Before Logstash had support for installable and upgradable plugins, Logstash provided a way to add additional paths to the ruby library search path in order to provide additional custom plugins.

Reasons I want to do this:

* Reduce complexity: We have two ways to provide plugins. That's one too-many.
* Reduce maintenance: We don't maintain or test the old-style plugins code paths, and deprecating this feature would reduce maintenance cost in this area.
* Future: As we move to allow pure-java plugins (keeping ruby!), I'd like start early and cut away some of the older code paths that won't be used once we land pure java plugin support.

Timing: It's too late for 5.0 to remove this feature, but we can deprecate it, but otherwise keep it working, in 5.1 and remove it completely in 6.0.

Impact and scope: This proposal only affects plugins that were previously loaded with Logstash 1.x/2.x `--plugin-path` or the `path.plugins` setting in 5.0. Gem plugin packages managed with `bin/logstash-plugin` are not impacted by this proposal.