Introduce a service that returns filtered plugin definitions

Problem/Motivation

Plugin discovery allows retrieval of plugin definitions.
Most plugin types support an alter on that list, but the result of those alters is cached and can only be used to fully remove a single plugin.

If the list of plugins needs to be filtered at runtime, it is currently up to the code that uses the definitions to support their own filtering.

Proposed resolution

Provide a unified mechanism for retrieving a filtered list of plugin definitions.

Remaining tasks

User interface changes

N/A

API changes

API addition

Data model changes

N/A