Added plugin base models and schema, parser models.

We already kinda described parsers like plugins via
MatchingParserInterface. This commits adds a Pydantic config
model called Parser (inheriting from Plugin) in front of
MatchingPerserInterface. This commit moves the interface
definitions from nomad.parsing.parsers into the config under
nomad.config.plugins. It also adds a simple schema plugin
model. But there are no existing schemas yet.

Changelog: Added