Refactor OffersMessageHandler Implementation to OffersMessageFlow

This commit introduces the `OffersMessageHandler` implementation
for `OffersMessageFlow`, enabling direct access to offer-specific
functionality through `OffersMessageFlow`.

With `OffersMessageFlow` now serving as the source of `OffersMessageHandler`
implementation, the implementation in `ChannelManager` is no longer needed
and has been safely removed.

Author: shaavan