Support reading age identity from file

[aabccd021]

It's a common security practice to read secrets from file instead of specifying it directly on configuration file / environment variable (e.g. GOOGLE_APPLICATION_CREDENTIALS=/run/secrets/gcs_backup_service_secret).

This PR enables litestream to read age identity from file path via identity-files attribute.

dbs:
  - path: /var/lib/db
    replicas:
      - url: s3://mybkt.litestream.io/db
        age:
          identity-files:
            - /path/to/age-secret-key
          recipients:
            - age1xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx