initial implementation

[yaauie]

Release notes

What does this PR do?

Implements the plugin mixin (see README.md)

Why is it important/What is the impact to the user?

Provides a path for plugins to ensure that a port is open during #register, and that they will be able to bind to it later in #run so that an unavailable port can be detected and raised in #register where it will affect pipeline status.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files (and/or docker env variables)
• I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

1. comment out the change to the gemspec
2. copy lib/logstash/plugin_mixins/port_management_support.rb into the same path in your local repo
3. run tests as normal

Related issues

• Relates: Logstash health_report API status not effected by pipeline errors elastic/logstash#17703

Use cases

• When a plugin cannot bind to a port, the issue should be raised in #register, which will cause the pipeline to fail to start, instead of in #run where the input itself will be restarted repeatedly.