Support providing a custom payment hash in the lightning-invoice invoice construction

[TheBlueMatt]

[ssbright]

Hello, I was unable to accomplish the previous PR I attempted but would like to try again. This seems like a good first issue for me so I will give it my best!

[dunxen]

This seems like a good first issue for me so I will give it my best!

Let me know if you're looking for any pre-review :)

[ssbright]

Thanks! I will @dunxen

[ssbright]

@dunxen I could use a pre-review please! I wrote must of the code, but my test is failing and I am unsure why.