1. Till now this UserConfig option was used to indicate whether the user
   wants to handles the payment for received Bolt12Invoices manually.
2. The future commits expand it's use case, so that it can also be used
   to indicate whether the user wants to handle response to the received
Invoice Request manually as well.
3. Rename this UserConfig appropriately to better represent its use
   case.

1. This event will be triggered when user enables manual handling of
   BOLT12 Messages.
2. With this event they can do some preprocessing on their end to make
   sure if they want to continue with this inbound payment.
3. For example, this can be used by the user to do the currency
   conversion on their end and respond to invoice request accordingly.

- Also move event generation after invoice request verification.

Introduce a new category of Bolt12 errors that contains the error
generated while trying to respond asynchronously to Bolt12 messages.

1. The amount_msats function first checks if any amount is given with
   the invoice requests, and defaults to using offer's amount if it's
absent.
2. This can be a problem if the amount in offer was represent as a
   Currency.
3. Update amount_msats to take in an optional custom amount which can be
   used in case amount if absent in Invoice Request.
4. This update will be utilised in the following commits to set
   custom amount_msats when responding to Invoice Request Asychronously.

Custom amount should only be provided if only the offer is denomiated
in currency, and no amount is provided in invoice request.

This is done so that the use of Custom Amount can be more specific to
the case when Offer is denomiated in Currency.

1. Introduce a set of private functions that handles the creation of
   response for a received Invoice Request.
2. This helps refactor the the logic of response creation in a single
   function.
3. This will be used in the following commit to send response for
   invoice_request asynchronously.