Deriving the offset from the LSN in SSE mode is using a backend implementation detail that could change

See: https://github.com/electric-sql/electric/pull/2869#pullrequestreview-2982441030

> I don't like the existing getOffset method that constructs one from the LSN and a _0 suffix. The offset being constructed from an LSN is a backend implementation detail we should not depend on. I think we had this in my original POC as a work around, but we really should replace this by adding the actual offset on the up-to-date message, at least in SSE mode.

I would advocate for adding the actual offset to all `up-to-date` messages, on both SSE and long polling mode, it seems like a sensible place to place it.