Give YAML plugin objects a useful `__str__`

Requested Update

Description

• Printing a YAML plugin doesn't tell you much at a glance, since it falls back to the full __repr__ dump of the plugin's contents. Add a __str__ to BaseYamlPlugin that gives a short readable summary for the cases where you just want to know which plugin you're looking at.

Background and Motivation

• This is the pluginify side of NRLMMD-GEOIPS/geoips#424, which covers the class-based plugins whose base class lives over in geoips. Both sides are using the same wording so the two kinds of plugin read the same way.
• __repr__ stays exactly as it is. The full dump is useful when you actually want to see everything in a plugin, and this is only about having a shorter option alongside it.
• Leaving family out of the output on purpose, since it's on its way to being deprecated.

Alternative Solutions

• Only fixing the class-based side in geoips and leaving this one alone. Workable, but then
  the two kinds of plugin behave differently for no good reason.

Environment

• https://github.com/NRLMMD-GEOIPS/geoips
• https://github.com/NRLMMD-GEOIPS/pluginify

Code to demonstrate issue

What you get now:

>>> print(products.get_plugin(("abi", "Infrared")))
ProductsPlugin({'interface': 'products', 'name': 'Infrared', 'spec': {...}, ...})

What we want, with repr still available for the full picture:

>>> print(products.get_plugin(("abi", "Infrared")))
Infrared (products plugin from geoips)

Checklist for Completion

• Add __str__ to BaseYamlPlugin
• Leave __repr__ alone
• Make sure it degrades gracefully when a plugin is missing attributes, so it never
  blows up on a half-built plugin in a debugger
• Keep the wording matching what geoips#424 produces
• Add tests
• Add a release note