feat(openvpn-tunnel): update certs duration and add certs regeneration functionality

[m-dilorenzi]

This pull request introduces new functionality for managing OpenVPN tunnel certificates, particularly enabling regeneration of server and client certificates, and improves new certificates validity to 10 years. The changes add a new script for regenerating certificates, expose certificate expiry information, and standardize EasyRSA environment variable usage across scripts.

Certificate Management Improvements

• Added a new script ns-openvpntunnel-regenerate-certs to regenerate server and client certificates for an OpenVPN tunnel, and updated the Makefile to install this script.
• Introduced the regenerate_tunnel_certs function and corresponding API endpoint (regenerate-server-certs) in ns.ovpntunnel to allow certificate regeneration via API calls.

Certificate Expiry Handling

• Implemented get_cert_expiry_ts utility and updated the tunnel listing API to include certificate expiry timestamps (cert_expiry_ts) in its responses when available.

Consistency and Cleanup in Certificate Scripts

• Standardized the setting of EasyRSA environment variables (such as EASYRSA_BATCH and EASYRSA_CERT_EXPIRE) in all relevant certificate management scripts to ensure consistent certificate creation and renewal.

These changes improve the maintainability and usability of OpenVPN tunnel certificate management, making it easier to automate certificate renewal and monitor certificate validity through the API.

Refs: #1481

[Tbaile]

Just bump package version and you're good to go