Supported protocols for `epm project:checkout` should be expendable

Instead of having two hard-coded methods to checkout a project from a remote server, this should be configurable.

The job accepts different types of project urls
The url protocol (http://, https://, ssh://, sftp://, webdav://, fbs:// ...) determines which checkout method is used
The handler for each protocol can be configured in the $cfg
The handler gets called with the project url and the target directory if set
The handler returns the project name and the target directory