Support mixing configurations

[tomblachut]

Inspired by #10612 I decided to tackle configuration bloat myself. Noticed that current Architect is being deprecated, but I believe this feature could land before 8.x.

Looking forward to getting feedback, never contributed to OSS before.

[ngbot]

Hi @tomblachut! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[hansl]

This is a great feature, but unfortunately we are re-working the architect API, so we won't let this in before the rework is done. The good news is that this feature can also be implemented after the rework and we should consider it for 8.0 (or 8.1). We'll let you know when this is ready. Cheers!

Please refer to #13463 for the PR of the new architect API.

[mgechev]

@tomblachut closing this PR since #13463 will change the architect API completely. Let's revisit once #13463 lands!

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