Enable filtering the data returned by extension

Problem/Motivation

Some use cases involving configuration updates require limiting actions to a particular subset of extensions. Currently, config_provider loads data from all extensions.

Proposed resolution

Introduce an $extensions argument to ConfigCollector::getInstallableConfig()that would limit configuration loaded to a specified set of extensions, defaulting to all if empty.

Provide a new method, ConfigProviderBase::listConfig(), that lists config by an optional set of extensions.

Remaining tasks

User interface changes

API changes

Data model changes