8927 plugin search

[arthanson]

Fixes: #8927

Refactors and adds to the search code to allow two things:

1. Allow plugins to have search capabilities
2. Make the search system 'pluggable' so a new Postgres search can be added.

The major changes are moving the search definitions to be class based (from the current dictionary), adding in a pluggable search backend system and making the search registration auto-scan the apps (which requires the class based definitions) to dynamically pick up new apps and plugins.

[jeremystretch]

This has gone through a few iterations, building on @arthanson's initial work, but ultimately I ended up settling on the following for this FR (with additional work remaining to be done under #10560):

• The SearchIndex class enables global search for each relevant model
• Indexes are registered to the application registry using the new register_search() decorator
• FilterSetSearchBackend retains the existing legacy global search functionality
• Add the search_indexes config parameter to PluginConfig

We'll undoubtedly need to keep adjusting the search backends implementation under #10560, but this feels like a solid start, and meets the primary objective of enabling global search for plugin models.