[Request]: Manage plugin properties via `container system property` subcommands.

[jglogan]

Feature or enhancement request details

Assumes we have moved property management to container system property per #604.

Users can run the other container system property subcommands to manage plugin properties as if they were core properties.

Possible approaches:

• Plugins that need new properties must implement system property themselves, and container delegates property management by shelling out to individual plugins. This makes it easy to implement custom validation, and reinforces the idea that each plugin can function as a complete, standalone "container-related tool", but requires more effort in the plugin.
• Plugins could advertise the properties they manage in metadata, and the DefaultsStore subsystem could change to enumerate plugins and incorporate their properties. Custom validation for setting properties would be more difficult.

In either case, the DefaultStore will need to be rewritten to make the property set programmable and more type-agnostic (see #608).

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