[Feature request] Add flow line variant with labels at the end

### Description

Currently, flow line labels are positioned in the middle of the line. It would be useful to have a variant (or property toggle) that places labels at the start or end of the flow line instead.

### Use case

End-positioned labels can be easier to read when flow lines overlap or when the annotation canvas is dense. They also pair better with certain layout patterns where the destination is the focal point.