proposal: package change metadata

We're working on [bodhi-scraper](https://github.com/RishabhSaini/bodhi-scraper#solution-what-does-bodhi-scraper-do) which is part of larger effort in [rpm-ostree](https://github.com/coreos/rpm-ostree/issues/4247#issuecomment-1467819269) to optimize packing images.

We require a [metadata file](https://github.com/fedora-infra/bodhi/pull/5172) (frequencyUpdateInfo.json) in the repodata of every (Fedora/RHEL/SCOS/Kionite)<Release> repository. This file will contain the list of all updates to all of the packages of the specific release.

These list of updates are more comprehensive than those present in `updateinfo.xml`.

We then combine the `frequencyUpdateInfo.json` of all the current and pending releases and process it to create a [file](https://github.com/RishabhSaini/bodhi-scraper#example-of-what-the-process_data-in-scriptsscraperpy-produces).

Since this file will be required for all rpm-ostree based Linux distributions, we wanted the architecture to integrate with `createrepo_c` to make the implementation more general.

Option 1: Inject this into `primary.xml`

Since this is a relatively small amount of additional metadata per package, we could add it to the primary package metadata. `primary.xml` is already enormous.

Option 2: Add a new `updatemeta.json`

We could introduce a new metadata file (JSON since this is 2020s) that contains this metadata instead.

Note that either option implies freezing (at least the first version of) the data shipped.