Containment: Add CustomEmbedded config value

This way each ContainmentType can be specified using the metadata

Otherwise, one has to specify a different value in the metadata and explicitly set it in the C++ code.
Ideally, this should have been a part of https://invent.kde.org/frameworks/plasma-framework/-/commit/84c29fa3d091a173d6e2e201262d61ac34dc6169,
but since we have only one usage, there is no point in backporting it to KF5