Will be useful when we later add support for static BOLT 12 invoices.

Will be useful when we support static BOLT 12 invoices.

Will be useful so the docs generated work for static invoices.

Useful for static invoice support.

Will be useful for static invoices.

Minor cleanup to be more concise.

Will be used in static invoices. Also test that we'll fail to decode if these
paths are included in single-use BOLT 12 invoices.

Define an interface for BOLT 12 static invoice messages. The underlying
format consists of the original bytes and the parsed contents.

The bytes are later needed for serialization. This is because it must
mirror all the offer TLV records, including unknown ones, which aren't
represented in the contents.

Invoices may be created from an offer.

Add a builder for creating static invoices for an offer. Building produces a
semantically valid static invoice for the offer, which can then be signed with
the key associated with the offer's signing pubkey.