feat(declarativeSettings): support encryption of sensitive values

[andrey18106]

• Resolves: Save passwords as sensitive values cloud-py-api/nc_py_api#344

Summary

This PR introduces new Declarative setting field attribute sensitive: true/false, available for Text and Password field types. Sensitive fields are encrypted on server side and the values are not exposed to the UI.

Declarative forms with handler (https://github.com/nextcloud/server/blob/master/lib/public/Settings/IDeclarativeSettingsFormWithHandlers.php) must implement this in their setValue, getValue methods, as well as external storage type (using events to handle getValue/setValue actions).

For ExApps AppAPI handles this automatically inside its event handlers.

TODO

• Clarify design of sensitive fields if they need explicit mark

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)

[provokateurin]

LGTM, just some suggestions for improvements.

[susnux]

LGTM except from the sprintf

[oleksandr-nc]

overall it looks good

[andrey18106]

/compile