To decouple offers and onion message-related code from `ChannelManager`,
this commit introduces the `message_received` function in `Offer/OnionMessageHandler`.

Currently, the function focuses on handling the retry logic for `InvoiceRequest`
messages. Moving this responsibility ensures a cleaner separation of concerns
and sets the foundation for managing offers/onion messages directly within the
appropriate handler.

Since `ChannelMessageHandler`'s `message_received` function was solely
used for handling invoice request retries. This function has been
removed from `ChannelMessageHandler`, and the relevant code has been
migrated to `OffersMessageHandler`'s `message_received`.

This ensures invoice request retries are now handled in the appropriate context.

This commit introduces a new struct, `OffersMessageFlow`,
to extract all offers message-related code out of `ChannelManager`.

By moving this logic into a dedicated struct, it creates
a foundation for separating responsibilities and sets up
a base for further code restructuring in subsequent commits.

A new trait, `OffersMessageCommons`, is introduced to encapsulate
functions that are commonly used by both BOLT12-related functionality
and other parts of `ChannelManager`.

This enables a clean transition of BOLT12 code to `OffersMessageFlow`
by moving shared functions into the new trait, ensuring they remain
accessible to both `ChannelManager` and the refactored BOLT12 code.

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

In subsequent commits, all other implementations of `OffersMessageHandler`
will be removed, consolidating `OffersMessageHandler` as the single
source of its implementation.

This commit introduces `OffersMessageFlow` in `functional_test`,
transitioning the source of `OffersMessageHandler` implementation
to `OffersMessageFlow`.

With this change, the `ChannelManager`'s implementation becomes
redundant and will be removed in the next commit.

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

- This commit introduces a new struct, `AnOffersMessageFlow`,
  which generically implements `OffersMessageFlow`.
- In subsequent commits, this struct will be utilized for
  documentation purposes.