Expose `advance_path_by_one`

[TheBlueMatt]

In #3132 we created advance_path_by_one to handle the case where are the intro node on a payment, but I believe LNDK may need that as well. We should check with them and then maybe expose it.

[orbitalturtle]

@TheBlueMatt Yes confirmed this would be helpful!

[TheBlueMatt]

Then I'll leave the milestone on it, though its somewhat optimistic it'll make it.

[valentinewallace]

@orbitalturtle Do you need to be able to advance blinded message paths as well, or just payment paths?

[orbitalturtle]

@valentinewallace Ah yup we need it for messages as well

[valentinewallace]

@valentinewallace Ah yup we need it for messages as well

Cool, added this to #3182.

[orbitalturtle]

@valentinewallace Thank you!