VerifySignature should be called in the BOLT11PaymentRequest ctor

As far as I can see, the signature is not verified by the constructor. Why not call `VerifySignature` iff `ExplicitPayeePubKey != null` and throw an exception if the signature does not match explicit pubkey?

Alternatively, the signature could also be verified in `Parse`/`TryParse`.

Locate the BOLT11PaymentRequest constructor and the VerifySignature, Parse, and TryParse entry points. Read how ExplicitPayeePubKey and signature errors are currently handled, then inspect existing tests around payment-request parsing or validation. Done means the chosen construction or parsing path verifies signatures when an explicit payee key is present and rejects mismatches.

Written by the indexing model from the issue text.