admin: Dry-run/preview mode

[mholt]

After troubleshooting on a call with a sponsor, I think it would be really helpful to have a "dry-run" or "preview" mode for the config API so that any request could be rehearsed before committing the change right away.

If there's any HTTP semantics for something like this, I'd be open to considering that.

Otherwise, maybe a custom HTTP header like Mode: Preview could be added:

$ curl -X POST \
	-H "Content-Type: application/json" \
	-H "Mode: preview" \
	-d '"DEBUG"' \
	"http://localhost:2019/config/logging/logs/default/level"

Whenever that header is present, the resulting config would be returned in the response body without applying it. It should come with an ETag (see #4579) to prevent race conditions.

[francislavoie]

Sounds related to #4537

[mholt]

Somewhat similar in end goal, but very different features.