Create plugin hooks for reporters

[travishathaway]

Related to: #13814

What's the idea?

In the aforementioned issue, we are creating a class structure that will eventually be used to customize the way output is rendered in conda. This issue will see us integrate this as a plugin hook, so that the output can be customize by plugin authors too.

We will create two new plugin hooks: one for "reporter handlers" and one for "output handlers". This will allow plugin authors to customize how output looks and where it is sent, respectively.

What should be done?

• Create a new conda_reporter_handler plugin hook. This will require authors to return a subclass of the ReporterHandler abstract base class.
• Create a new conda_output_handler plugin hook. This will require authors to return a subclass of the OutputHandler abstract base class
• Add new documentation for these two plugin hooks
• Refactor our existing reporting handlers (console and json) to use the plugin system
• Refactor our existing output handlers (stdout) to use the plugin system