Long record values need to be splitted

### Explanation There's a length limit for record values, defined in ([RFC 1035, Section 3.3](https://datatracker.ietf.org/doc/html/rfc1035#section-3.3)) > \<character-string> is a single length octet followed by that number of characters. <character-string> is treated as binary information, and can be up to 256 characters in length (including the length octet). This is especially important for DKIM TXT records, but can also happen with other record types. Some nameservers do not start when the zonefile contains a too long record. ### Reproducer: ```nix { zones."example,net"."mail._domainkey".txt.data = "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2WJ46bl9UqBY9ZxqkVCBdSiysIJMUbWS3BK10Lupe4T5+jWAcdzJraznWeaVF/mR/9TyiB7lE79ZB6WxHxTwwJ5UZjURwImKAKqSGPXPACIj+LHyx5j2nHN4CawC6bkCmpGT99B7I/5bCelekoAHV9U/4pE2YEjgA0VxvlSKHB2Y7cPWL303DInYGaTrvMczuwLYoEwIiBirffYNqHyrOJE9A+ZQRdLjM8DFOxegAOV9mcHb3MwneJuu86Czz45UIrQ7AxkMUNKgHitqTSnXzLWd4BF6Kf3XUh/lED7WPdviBLJo/1H0Cgch8RRlinTeDVliHDQ6/zLWpk6+k3iKkQIDAQAB; s=*;"; } ``` outputs following zone file: ``` mail._domainkey.example.net. IN 86400 TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2WJ46bl9UqBY9ZxqkVCBdSiysIJMUbWS3BK10Lupe4T5+jWAcdzJraznWeaVF/mR/9TyiB7lE79ZB6WxHxTwwJ5UZjURwImKAKqSGPXPACIj+LHyx5j2nHN4CawC6bkCmpGT99B7I/5bCelekoAHV9U/4pE2YEjgA0VxvlSKHB2Y7cPWL303DInYGaTrvMczuwLYoEwIiBirffYNqHyrOJE9A+ZQRdLjM8DFOxegAOV9mcHb3MwneJuu86Czz45UIrQ7AxkMUNKgHitqTSnXzLWd4BF6Kf3XUh/lED7WPdviBLJo/1H0Cgch8RRlinTeDVliHDQ6/zLWpk6+k3iKkQIDAQAB; s=*;" ``` ### Expected output ``` mail._domainkey.example.net. IN 86400 TXT "v=DKIM1; k=rsa; p=MIIBIjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEA2WJ46bl9UqBY9ZxqkVCBdSiysIJMUbWS3BK10Lupe4T5+jWAcdzJraznWeaVF/mR/9TyiB7lE79ZB6WxHxTwwJ5UZjURwImKAKqSGPXPACIj+LHyx5j2nHN4CawC6bkCmpGT99B7I/5bCelekoAHV9U/4pE2YEjgA0VxvlSKHB2Y7cPWL303DInYGaTrvMczu" "wLYoEwIiBirffYNqHyrOJE9A+ZQRdLjM8DFOxegAOV9mcHb3MwneJuu86Czz45UIrQ7AxkMUNKgHitqTSnXzLWd4BF6Kf3XUh/lED7WPdviBLJo/1H0Cgch8RRlinTeDVliHDQ6/zLWpk6+k3iKkQIDAQAB; s=*;" ```