Disambiguate blinded path `ForwardNode`s between payment + message

[TheBlueMatt]

We currently have two structs with identical names in our public API - blinded_path::message::ForwardNode and blinded_path::payment::ForwardNode. This makes the API somewhat awkward to use - users have to try (and fail) to import ForwardNode twice only to then have to change their imports.

More importantly, however, this makes the API very hard to use in some bindings languages where rename-imports or module imports aren't available.

Thus, here, we rename both to give them context.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 91.48936% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.79%. Comparing base (bbfa15e) to head (2ce2fe9). Report is 10 commits behind head on main.

Files	Patch %	Lines
lightning/src/onion_message/messenger.rs	50.00%	3 Missing :warning:
lightning/src/util/test_utils.rs	50.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3265 +/- ## ========================================== - Coverage 89.82% 89.79% -0.03% ========================================== Files 125 125 Lines 102867 102867 Branches 102867 102867 ========================================== - Hits 92398 92370 -28 - Misses 7753 7779 +26 - Partials 2716 2718 +2 ```

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[jkczyz]

Looks like fuzz code needs to be updated.

[TheBlueMatt]

Fixed.