[loki] Configuration for sigv4 authentication in the ruler being ignored

[taharah]

Describe the bug
Currently, the sigv4 authentication configuration in the Ruler is being overwritten, and ignored, when the per tenant overrides are merged in (see https://github.com/grafana/loki/blob/v2.5.0/pkg/ruler/registry.go#L232). As a result, there is no way to configure sigv4 authentication for remote writes. Remote writes fail with the error:

err="server returned HTTP status 403 Forbidden: {\"message\":\"Missing Authentication Token\"}"

To Reproduce
Steps to reproduce the behavior:

1. Started Loki v2.5.0
2. Configured Ruler to use sigv4 authentication for the remote write URL
3. Create a recording rule

Expected behavior
Expected the sigv4 authentication configuration to be passed into the client configuration.

Environment:

• Infrastructure: k8s
• Deployment tool: helm

Screenshots, Promtail config, or terminal output
If applicable, add any output to help explain your problem.