Offers deprecate string recurrence

[rustyrussell]

@cdecker points out that it's ugly to have a string variant, and I agree. Deprecate now (quickly, it's experimental) so GRPC/Rust bindings don't have to support it!

[cdecker]

This is missing the update to the docs/schema/offer.schema.json which is where the issue arises in the first place when trying to get it under msggen coverage. I'll add that and then merge.

In the meantime, thank you very much for this @rustyrussell ^^

[cdecker]

Turns out this doesn't address the schema issues we have:

FAILED tests/test_pay.py::test_fetchinvoice_recurrence - jsonschema.exceptions.ValidationError: '1minutes' is not of type 'object'

Failed validating 'type' in schema['properties']['recurrence']:
    {'properties': {'period': {'description': 'the positive number which '
                                              'the time unit refers to',
                               'type': 'u32'},
                    'time_unit': {'description': 'a positive number '
                                                 'referring to a time unit '
                                                 '- 0 (seconds), 1 (days), '
                                                 '2 (months), 3 (years)',
                                  'type': 'u32'},
                    'time_unit_name': {'description': 'the name of the '
                                                      'specified time unit '
                                                      '- either  '
                                                      '"seconds", '
                                                      '"minutes", "hours", '
                                                      '"days", "weeks" or '
                                                      '"years"',
                                       'type': 'string'}},
     'required': ['time_unit', 'period'],
     'type': 'object'}

On instance['recurrence']:
    '1minutes'

This error is caused in #6896 after applying this patch. We need to remove the shorthand usage of 1minutes in the tests in order to fix this.

[cdecker]

Nevertheless, this is a good change, let's merge it asap, and then address that request schema issue, using redundant argument encodings.

[rustyrussell]

That's different. That json schema is wrong: recurrence is a string, never a raw number.

If we wanted, we could make it an object, in which case we would use a number and a string (an enum in JSON terms). But using the draft bolt encoding here is wrong, since it's highly experimental and subject to change.

TBH, I would not support recurrence in the GRPC API for now.

[vincenzopalazzo]

ACK 9d7055d

The CI isssue was already reported as a issue, and looks like not related to this PR #7043