* DEV: Add framework for filtered plugin registers
Plugins often need to add values to a list, and we need to filter those lists at runtime to ignore values from disabled plugins. This commit provides a re-usable way to do that, which should make it easier to add new registers in future, and also reduce repeated code.
Follow-up commits will migrate existing registers to use this new system
* DEV: Migrate user and group custom field APIs to plugin registry
This gives us a consistent system for checking plugin enabled state, so we are repeating less logic. API changes are backwards compatible
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