Provide the signer with a full `RawBolt11Invoice` to sign

Provide the signer with a full `RawBolt11Invoice` to sign

Now that the `lightning` crate depends on the `lightning-invoice`
crate, there's no reason to have the `sign_invoice` method take raw
base32 field elements as we can now give it a real
`RawBolt11Invoice`, which we do here.

This simplifies the interface and avoids a
serialization-deserialization roundtrip when signing invoices in a
validating signer.

FIxes #3227