Allow Asynchronously responding to Invoice Request

[shaavan]

Introduce a new event and set of supporting functions to allow flexibility in responding asynchronously to a received invoice request.

This PR provides foundational work for further improvements in future. For example, this allows supporting Offers created in currency denomination, as a user can do appropriate pre-processing for a received InvoiceRequest, before sending an Invoice for it.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 74.01575% with 66 lines in your changes missing coverage. Please review.

Project coverage is 89.57%. Comparing base (66fb520) to head (9b4daa4).

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	74.82%	34 Missing and 1 partial :warning:
lightning/src/events/mod.rs	0.00%	19 Missing :warning:
lightning/src/offers/invoice.rs	73.07%	4 Missing and 3 partials :warning:
lightning/src/offers/invoice_request.rs	57.14%	3 Missing :warning:
lightning/src/ln/offers_tests.rs	96.72%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3318 +/- ## ========================================== - Coverage 89.64% 89.57% -0.07% ========================================== Files 126 126 Lines 102676 102849 +173 Branches 102676 102849 +173 ========================================== + Hits 92045 92132 +87 - Misses 7912 7987 +75 - Partials 2719 2730 +11 ```

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[shaavan]

Updated from pr3318.01 to pr3318.02 (diff):

Changes:

1. Rebase on main to resolve merge conflicts and fix CI.

[shaavan]

Updated from pr3318.02 to pr3318.03 (diff): Addressed @jkczyz comments

Changes:

1. Update Responder to be required field.
2. Update InvoiceRequestReceived Docs.
3. Update condition check when the Custom Amount should be provided.
4. Create the InvoiceRequestReceived event only when InvoiceRequest is verified.