Provide a JSON:API field enhancer and processor for rrules

Problem/Motivation

When exposing content that has recurring dates, either with JSON:API or the REST export display of a View, just the ID number of the rrule is provided, like so:

{"value":"2022-09-11T17:00:00+00:00","end_value":"2022-09-11T22:30:00+00:00","duration":330,"rrule":29250,"rrule_index":1,"timezone":"America\/Chicago","format":"Y-m-d\\TH:i:sP"}

Since a rule with that ID number and the correct value is unlikely to already exist in the destination site, we need more information about the rule. Ideally we would have both the UUID (so we can look it up if it already exists) and the text of the rule itself (so we can create it if it doesn't); even better if we had the list of instances.

It is possible to make a View of just Smart Date Recurring Rules and make a REST export of that. You get something like this:

{"rid":[{"value":3597}],"uuid":[{"value":"0006406d-5b8f-4685-b68c-93db485de07c"}],"rule":[{"value":"RRULE:FREQ=DAILY;INTERVAL=1;UNTIL=20210402T164559Z"}],"freq":[{"value":"DAILY"}],"limit":[{"value":"UNTIL=20210402T164559Z"}],"parameters":[],"unlimited":[{"value":true}],"entity_type":[{"value":"node"}],"bundle":[{"value":"opportunity"}],"field_name":[{"value":"field_dates"}],"start":[{"value":"2021-04-01T14:00:00+00:00","format":"Y-m-d\\TH:i:sP"}],"end":[{"value":"2021-04-01T21:45:59+00:00","format":"Y-m-d\\TH:i:sP"}],"instances":[{"data":{"1":{"value":1617285600,"end_value":"1617313559"},"2":{"value":"1617372000","end_value":"1617399959"}}}]}

That is sufficient info to import the rule. However, there's no way to filter the View based on where the rule is in use - there are no relationships defined. So if you don't want to import all the rules, you would have to query for each one individually by its ID — as you are importing the nodes, so that you can create the rule and get its local ID number before you save the values of the recurring date field itself.

If there were a JSON:API endpoint for rules, then they could be ?included → in the request for the content, and all the info would be in one place.

Proposed resolution

• Make a JSON:API endpoint at /jsonapi/smart_date_rule that outputs the same thing you currently get by making a View of rules and displaying it as REST export.
• Make the serialization of any recurring date field show the rule as a reference instead of just an integer value.
• Make it work with the ?include parameter, for example /jsonapi/node/event?include=field_dates.rule .

Remaining tasks

1. Figure out how to do the things.
2. Do the things.