set-siteconfig has historically been useful only for basic setting

management. It wasn't possible to set multiple settings, add new

settings, delete settings (restoring to defaults), or manipulating

lists. This made it very difficult to perform some kinds of settings

management without diving into Python code or modifying the database

more directly.
Now, set-siteconfig can take either a JSON Patch or a JSON Merge

Patch, either as a string or read from STDIN. It'll apply to the

database, show a diff of the resulting settings, and prompt the user for

confirmation before applying the changes. This prompt is a safeguard in

case a patch does something unexpected, and can be disabled by passing

--confirm (which is useful for automation).
--dry-run can also be used to simulate making a change without

actually saving the new settings, helping people to test their patches.
As a safeguard, patches that completely wipe settings can't be applied.

It's worth pointing out, though, that there are no safeguards against

applying invalid settings, a siteconfig settings do not currently offer

validation. That leaves this as an advanced feature that must be used

carefully.
Setting individual keys using --key/--value is still supported and

works as before. These do not provide validation, but they will show the

differences between settings. They do now respect the new --dry-run

setting.

Tested both patching options and the --key/--value options. In each

case, I tested:

Setting the existing values (no changes to save)
With --dry-run
With --confirm
With manual prompting (and all Yes/No options), for patching.

Verified that diffing always looked correct and that settings applied

when confirmed.