Support 3-way configuration merging

Problem/Motivation

Merging in configuration updates without overriding customizations requires 3-way configuration merges.

For the configuration staging workflow, the three configuration states are:

• Configuration item as previously snapshotted
• Configuration item as staged
• Configuration item as saved in the active configuration storage

For configuration updates from extensions, the states are:

• Configuration item as previously snapshotted; see #2960870: Snapshot configuration as installed and updated from recipes and extensions →
• Configuration item as provided by updated version of extension (e.g., in config/install)
• Configuration item as saved in the active configuration storage

The Config Merge → in contrib includes a 3-way merging method. The module also provides a Config Filter plugin to allow safely merging into live using the staging workflow.

Proposed resolution

Adapt and add ConfigMerger along with accompanying test coverage.

Remaining tasks

User interface changes

API changes

Data model changes