The function used to sign BOLT 12 messages only takes a message digest.
This doesn't allow signers to independently verify the message before
signing nor does it allow them to derive the necessary signing keys, if
needed.

Introduce a TaggedHash wrapper for a message digest, which each unsigned
BOLT 12 message type constructs upon initialization. Change the signing
function to take AsRef<TaggedHash>, which each unsigned type implements.
This allows the signing function to take any unsigned message and obtain
its tagged hash.

InvoiceBuilder is parameterized by a SigningPubkeyStrategy, either
ExplicitSigningPubkey and DerivedSigningPubkey. It also holds an
Option<KeyPair>, which may be None and Some for those strategies,
respectively. This leads to methods for InvoiceBuilder parameterized by
DerivedSigningPubkey needing to blindly unwrap the Option<KeyPair>.
Instead, have DerivedSigningPubkey wrap KeyPair.

Using `contents` for the field name is more consistent with the signed
messages.

InvoiceRequest wraps OfferContents, which shouldn't be exposed as it is
an implementation detail. Define a macro for Offer accessor methods so
that InvoiceRequest and UnsignedInvoiceRequest can also define them.

Various messages wrap InvoiceRequestContents, which shouldn't be exposed
as it is an implementation detail. Define a macro for InvoiceRequest
accessor methods so that these messages can also define them.

Also, expose both Offer and InvoiceRequest functions in
UnsignedInvoiceRequest.

Bolt12Invoice can either be for an Offer (via an InvoiceRequest) or a
Refund. It wraps those types, so expose their methods on both
Bolt12Invoice and UnsignedBolt12Invoice.

Since Refund does not have all the Offer/InvoiceRequest methods, use an
Option return type such that None can returned for refund-based
invoices.

For methods that are duplicated between Offer/InvoiceRequest and
Bolt12Invoice, prefer the (non-Option, if applicable) method from
Bolt12Invoice (e.g., amount_msats, signing_pubkey).

An earlier commit introduced TaggedHash for use in sign_message. For
consistency, use it in verify_signature, too.

BOLT 12 messages need to be signed in the following scenarios:
- constructing an InvoiceRequest after scanning an Offer,
- constructing an Invoice after scanning a Refund, and
- constructing an Invoice when handling an InvoiceRequest.

Extend the NodeSigner trait to support signing BOLT 12 invoices such
that it can be used in the latter contexts. The method could be used
in an OffersMessageHandler.