feat: add support for custom expiry

[louneskmt]

A quick PR adding support for custom invoice expiry delta in seconds. By default, it was set to 3600 (one hour). It is now possible to add an optional parameter to change this value.

The API and internal functions are the only scopes of this PR, a follow-up PR would be needed to add CLI and Web support.

@johncantrell97 Do I need to add an expiry column to the Payment Model?

[johncantrell97]

not sure if we do this other places with time but I personally prefer to know the units similar to amt_msat. When I look at expiry the first thing I'll ask is if it's in milliseconds or seconds. how do you feel about naming it expiry_secs?

[louneskmt]

Sounds great to me 👌

[johncantrell97]

Hm, I guess we could add it explicitly to the payments table. Technically it's encoded in the invoice which is already included in the table, right? I guess some of the other fields are probably also already encoded in the invoice and added seperately as well.

Let's add it. Don't bother with a new migration though, just add it to the create_payments_table one and then you'll need to pass it through when we create an invoice.