Porch API calls should be atomic

[kispaljr]

Right now if processing an API call in the apiserver fails somewhere in the middle, then the modifications done before the failure are not rolled back.

For example here: https://github.com/nephio-project/porch/blob/cb1b392069de7711bb29c3524d6fc464e991f82a/pkg/engine/engine.go#L131 :

During PackageRevision creation, if creating the internal PackageRev object fails, the whole operation returns with an error (as it should), but the creation of a new branch already happened in the git repo and it is not rolled back. That will prevent retrying the creation of the PackageRevision, because it will detect a conflict in the workspace name, because of git's state, even though there is no other PackageRevision object exists with a conflicting workspace name.

The example is about the "Create Package Revision" call, but the same is true for other API calls as well.

Proposal: make sure that on error all previous changes are reverted before returning with the error, for all API calls in the porch apiserver.