backstage release v0.91.0

Added

• ChartPicker custom scaffolder field.
• ChartTagPicker custom scaffolder field.
• YamlValuesEditor custom scaffolder field.
• YamlValuesValidation custom scaffolder field.
• GS backend plugin.
• Container Registry frontend API client and backend service.

Changed

• helmcharts catalog entity annotation format has been changed. See ./docs/releases/v0.91.0-changelog.md for more information.