Fix schema for V0 HttpConfig headers

Currently, the field Headers is a `map[string]Header` where value is the following type https://github.com/grafana/alerting/blob/11bc3c6bc5f066f5025c1aab9cb17cb033482ef5/http/v0mimir1/config.go#L318-L322

The schema definition is not correct as it represents the value as just `map[string]string`.

This introduces two problems:
1. Secrets in headers are not properly discovered\masked
2. Headers are not properly displayed.

Start with http/v0mimir1/config.go, especially the Header type referenced in the issue, then locate the schema definition for V0 HttpConfig.Headers. Verify that the schema represents header values with the same structure as the Go type, and confirm that header secrets are discovered or masked and headers are displayed correctly.

Written by the indexing model from the issue text.