In order to generate and InvoiceSent event, it would be cleaner to have
one location where a Bolt12Invoice is successfully generated. Refactor
the handling code to this end and clean-up line length by making some of
the type conversions more streamlined.

In order to provide an InvoiceGenerated event, it would be cleaner to
have one location where a Bolt12Invoice is successfully created.
Refactor the handling code to this end and clean-up line length by
making some of the type conversions more streamlined.

Bolt12Invoice will be added to a new Event::InvoiceGenerated variant.
These traits along with PartialEq are required to be implemented for any
type used in an Event.

When sending an invoice for a refund, information from the invoice may
be useful for caller. For instance, the payment_hash can be used to
track whether the refund was paid. Return the invoice to facilitate this
use case.

Providing LDK-specific data in a BlindedPath allows for the data to be
received with the corresponding payment. This is useful as it can then
be surfaced in PaymentClaimable events where the user may correlated
with an Offer, for instance. Define a PaymentContext enum for including
various context (e.g., offer, refund, static invoice).

PendingInboundPayment::BlindedReceive includes a PaymentContext, which
the recipient includes in each blinded path. Included this when
processing HTLCs in ChannelManager, first into PendingHTLCRouting and
then to OnionPayload. Later, this will be included in the PaymentPurpose
when surfaced to user in PaymentClaimable and PaymentClaimed events.

Use a merkle root hash of the offer TLV records to define an offer id.
Will be included in a BOLT 12 invoice's blinded payment paths in order
for the recipient to identify which offer the payment is for.

Extract the OfferId from the offer metadata sent back in the
InvoiceRequest and include it in VerifiedInvoiceRequest. This can be
used to correspond the eventual payment for the invoice response by
including it in the invoice's blinded payment paths.

When constructing blinded payment paths for a Bolt12Invoice response,
include the OfferId so that the eventual payment can be correlated with
the Offer.

When requesting a payment for a refund, include a context in the
Bolt12Invoice's blinded payment paths indicated it is for such. When the
eventual payment is received, the user can use the payment hash to
correlate it with the corresponding Refund.

PaymentPurpose will be expanded to include BOLT 12 payment context.
Rename the InvoicePayment variant accordinly before new variants are
added.

In order to provide more context in PaymentClaimable and PaymentClaimed
events, introduce new variants of PaymentPurpose for use with BOLT 12
payments. Separate variants are used for offers and refunds. An unknown
variant is used for backwards compatibility and ease of testing, but is
otherwise not publicly constructable.

When constructing a PaymentPurpose in ChannelManager, use the
PaymentContext from OnionPayload to determine which variant to
construct, including those for BOLT 12 payments.

When receiving a payment, it's useful to know information about the
InvoiceRequest. Include this data in PaymentContext::Bolt12Offer so
users can display information about an inbound payment (e.g., the payer
note).

Clarify that payment_preimage should only be used to claim a payment
when handling Event::PaymentClaimable, not Event::PaymentClaimed.