feat: add retries for s3 ObjectExists calls

[paul1r]

What this PR does / why we need it:
It was determined via careful examination that Loki would issue retries for S3 GetObject calls, but would not retry calls to ObjectExists.

For transient issues (e.g. rate-limiting), it makes sense to retry the command to see if an object does exist. If the object is not found (meaning, a 404 return code, and a successful query of the S3 storage to show the object does not exist), the retries are not leveraged, as it is not needed.

Which issue(s) this PR fixes:

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
  • Note that Promtail is considered to be feature complete, and future development for logs collection will be in Grafana Alloy. As such, feat PRs are unlikely to be accepted unless a case can be made for the feature actually being a bug fix to existing behavior.
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR