Allow a wse_config entity to represent a config partial rather than a complete config entity

Problem/Motivation

Experience Builder (XB) isn't yet using Workspaces, but that's on our pre-1.0 roadmap. Per #3493461: Identify roadblocks to staging config in Workspaces (e.g., via wse_config), when we add Workspaces integration to XB, we'll do it for XB config entities as well as content.

One of our config entities is page_template which is a single config entity containing configuration for every region in the theme. However, we potentially want to support two different content editors editing two different regions in two different workspaces. (Related, see #3500390: The pending changes API endpoint should list individual regions for global template changes for the pre-Workspaces implementation.)

Is this a unique use case (if so, perhaps we should refactor our page_template entity into per-region entities), or have there been similar use cases for e.g., Views displays, etc.? If the latter, would there be interest in an MR that implements the concept of a wse_config entity representing a sub-portion of a config entity?

Steps to reproduce

Proposed resolution

TBD depending on whether there's even interest in this.

Remaining tasks

User interface changes

API changes

Data model changes