fix(Router): add setting payer_is_user= kwarg for plan

[fubuloubu]

What I did

Noticed that it wasn't really possible to do a swap with Safe (in a simulation context), so adding this kwarg allows the pass-through of the critical payer_is_user= kwarg so you can simulate a swap with a Safe by first sending tokens to the Universal Router (NOTE: this should NOT be used for production, as your tokens will disappear)

I tried testing it using the propose_from_simulation capability of ape-safe, but realized for some use cases it wouldn't work very well in practice when handling the values given after the swap (but you can do a swap with this now!)

How I did it

How to verify it

Checklist

• Passes all linting checks (pre-commit and CI jobs)
• New test cases have been added and are passing
• Documentation has been updated
• PR title follows Conventional Commit standard (will be automatically included in the changelog)