Add a configurable option in Plesk to retain the existing private key when re-issuing or renewing a Let's Encrypt SSL certificate.

This would be useful in environments where DANE/TLSA support is enabled. Retaining the same private key ensures that TLSA records (e.g., type 3 1 1) remain valid across certificate renewals. This is critical for maintaining secure SMTP delivery and avoiding disruption in DNSSEC-enabled domains.