security/acme-client: Add GCore DNS Provider

[olofvndrhr]

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• I have read the contributing guide lines at https://github.com/opnsense/plugins/blob/master/CONTRIBUTING.md
• I have searched the existing issues, open and closed, and I'm convinced that mine is new.
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I'm currently using GCore Managed DNS for my domains, and would like to automate the certificate renewal on the OPNsense firewall itself as opposed to an external tool.

The implementation in the acme.sh script is already there: https://github.com/acmesh-official/acme.sh/blob/master/dnsapi/dns_gcore.sh

Best regards