Add mergedYaml to CiConfigResolver

What does this MR do?

• Adds mergedYaml to the response in CiConfigResolver

query ($content: String!, $projectPath: ID!) {
  ciConfig(content: $content, projectPath: $projectPath) {
    mergedYaml
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides