[Bug] When chosing to input a KeyPackage, `self_update` creates an Update proposal

[kkohbrok]

The self_update function of the mls_group API takes an optional KeyPackage as input. When that happens, the function creates an update proposal from that KeyPackage and tries to commit it. However, committing update proposals with one's own KeyPackages is not allowed and the path must be used instead.

For the function to change this behaviour, the CreateCommitParams struct needs to allow the caller to pass in an explicit KeyPackage, which can then be used as basis for the path. Also, a test should be added that actually does this.