Read openmetrics of different types

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.

Fixes #11 (closed)