Invoice request message interface and data format

Invoice request message interface and data format

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

The bytes are later needed when constructing an `invoice` message. This
is because it must mirror all the `offer` and `invoice_request` TLV
records, including unknown ones, which aren't represented in the
contents.

The contents will be used in `invoice` messages to avoid duplication.
Some fields while required in a typical user-pays-merchant flow may not
be necessary in the merchant-pays-user flow (e.g., refund, ATM).