api: jsonrpc api `get_all_ui_config_keys` to get all "ui.*" config keys

[Simon-Laux]

Adds an api to get all ui config keys. There already is an option to get all normal config keys ("sys.config_keys"), but before this pr there was no way to get all ui.* config keys.

Why is this api needed?

For webxdc cleanup on desktop, which stores window position in a ui config key (such as ui.desktop.webxdcBounds.676464) as soon as a webxdc is opened since many versions now. So listing all ui keys is a good way for us to find out which webxdc may have web data stored. unfortunately electron does not (yet?) have a way to list all origins that have web-data like android does, so this is the next best thing we can do before itterating all possible ids, see also https://github.com/deltachat/deltachat-desktop/issues/5758.

Why is this only a jsonrpc api and not another special/virtual config key like "sys.config_keys"?

r10s indicated that ui.*-config keys are barely used (https://github.com/deltachat/deltachat-desktop/issues/5790#issuecomment-3598512802), so I thought it makes more sense to add it as dedicated api which's existentence is checked by the typechecker, so it will be easier to not miss it when we should remove the api again in the future.

But we could also do a dedicated special/virtual config key for it, if you think that is better, this is easy to change.