Rename feature_flag.system back to feature_flag.provider_name

[lmolkova]

Extracted from #1613

Given uncertainty around *.system attribute, I'd like to revert recent unreleased changes to feature_flag.system and rename it back to feature_flag.provider_name. This would reduce number of breaking changes.

We can't call it feature_flag.provider.name yet due to const name collision with existing ``feature_flag.provider_name` - #1462

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[dyladan]

Should I assume this is a temporary reversion until a final decision is made regarding #1581 et alia?

[lmolkova]

Should I assume this is a temporary reversion until a final decision is made regarding #1581 et alia?

yes, this is temporary until we have a final consistent pattern.