feat: use DNS certificate if available for staging and production

[timbru31]

This pull request includes several updates to configuration files and deployment workflows to enhance security and streamline the deployment process. The most important changes include adding new environment variables, updating GitHub workflow files, and modifying the serverless.yml configuration for better environment handling and security.

Environment Variables:

• Added FRONTEND_CERTIFICATE_ARN and BACKEND_CERTIFICATE_ARN to .env.example for managing SSL certificates.

GitHub Workflow Updates:

• Updated .github/workflows/merge-to-master.yml to include the timbru31-patch-1 branch for deployment.
• Added new secrets (HOSTED_ZONE_ID, FRONTEND_CERTIFICATE_ARN, BACKEND_CERTIFICATE_ARN) to the jobs section in .github/workflows/reusable-deploy-job.yml for deployment configuration.
• Added a step to set environment variables without HTTPS in .github/workflows/reusable-deploy-job.yml.

Serverless Configuration:

• Introduced conditions to check if the environment is production or staging in serverless.yml.
• Updated CloudFront distribution configurations to include SSL certificate management and response headers policy. [1] [2]
• Added Route53 alias records for frontend and backend distributions in serverless.yml.
• Split the S3 bucket policy into separate policies for frontend and backend buckets in serverless.yml. [1] [2]