OOTB: Create a modular settings system

**Related to:** bottlerocket-os/bottlerocket#2669

When creating an out-of-tree variant, maintainers should have the ability to provide changes to the settings system. As such, the settings system needs to be modified to allow modular addition and modification to the settings API.

The design for this is proposed in bottlerocket-os/bottlerocket#2834. This issue is to track implementation of this system.

Start by reading the design proposal in bottlerocket-os/bottlerocket#2834 and the related settings-system issue in bottlerocket-os/bottlerocket#2669. No files, tests, or entry points are named here; completion means implementing the proposed modular additions and modifications to the settings API for out-of-tree variants.

Written by the indexing model from the issue text.