Establishes a relationship between key and secret internally in the setting which may be retrieved later.
Before 1.24, secret must not be null and not an empty string. Since 1.24, secret
can be set to an empty string. It can also be set to null to unset the key. In that case, the
behavior is as if calling remove_secret.
| this |
the SettingVpn |
| key |
a name that uniquely identifies the given secret |
| secret |
the secret to be referenced by |