DBus/SecretService: Support "plain" algorithm as well

Some applications use a hardcoded "plain" algorithm for secret transfer and the spec says: "It is strongly recommended that a service implementing this API support the plain algorithm." https://specifications.freedesktop.org/secret-service/latest/ch07s02.html

In https://invent.kde.org/frameworks/kwallet/-/blob/master/src/runtime/kwalletd/kwalletfreedesktopservice.cpp#L265 support is limited to dh-ietf1024-sha256-aes128-cbc-pkcs7.

It would be awesome if "plain" would also be supported to support more applications.

FYI: @ptrnine @nicolasfella