arch_release_promotion/release.py:
Add `Metric`, `SizeMetric`, `AmountMetric` and |VersionMetric| models,
that track metrics with size, amount or version (respectively).
Change the `Release` model to use `amount_metrics`, `size_metrics` and
`version_metrics` attributes instead of `info`.

arch_release_promotion/config.py:
Change the `ReleaseConfig` model to track `version_metrics`,
`size_metrics` and `amount_metrics` instead of `info_metrics`.

arch_release_promotion/files.py:
Change `read_metrics_file()` to return a tuple of AmountMetric,
SizeMetric and VersionMetric lists and read the provided file only if it
exists.

arch_release_promotion/cli.py:
Change `main()` to correctly initialize instances of `Release` with the
different types of metrics.

tests/*:
Change the tests to match the changes in signature and attributes.