[PP-2] Port SDC to use ExtensionTypeInterface

Problem/Motivation

At one point 📌 Introduce ExtensionTypeInterface with public constants for extension types Needs work was considered a blocker for ✨ Add Single Directory Components as a new experimental module Fixed . While working on #3350946 we realize it's more complicated than it seems, we need to work out the right scope for converting things to using it, add other, similar interfaces with other constants, etc. While all that dust settles, we can unblock #3340712 with @todo comments pointing here.

Steps to reproduce

Proposed resolution

Remaining tasks

1. Land 📌 Introduce ExtensionTypeInterface with public constants for extension types Needs work
2. Land ✨ Add Single Directory Components as a new experimental module Fixed
3. Port SDC to use ExtensionTypeInterface
4. Reviews / refinements
5. RTBC

User interface changes

API changes

Data model changes

Release notes snippet