Provide special handling of alters when snapshotting

Problem/Motivation

When snapshotting, we need to apply only the specific alters provided by the extensions we're snapshotting. When fetching available updates, we need to apply all alters from all installed modules.

Proposed resolution

• Add new interface SyncConfigProviderInterface extending ConfigProviderInterface and adding a public method ::alterConfigForSnapshotting().
• Add new interface SyncConfigCollectorInterface extending ConfigCollectorInterface and adding public methods ::addConfigForSnapshotting() and ::alterConfigForSnapshotting().
• Implement SyncConfigCollectorInterface, providing methods to be called in snapshotting.

Remaining tasks

User interface changes

API changes

Data model changes