EXAMPLES: lua/acme: add a dns-01 handler for Gandi LiveDNS API

EXAMPLES: lua/acme: add a dns-01 handler for Gandi LiveDNS API

This Lua script automates dns-01 ACME challenges using the Gandi LiveDNS
API v5. It subscribes to the ACME_DEPLOY event to set the required
_acme-challenge TXT record via the Gandi REST API, signals HAProxy that
the challenge is ready using ACME.challenge_ready(), then cleans up the
TXT record once the certificate is issued on ACME_NEWCERT.

The API key is read from the GANDI_API_KEY environment variable at
startup. Zone discovery is automatic: the script probes parent zones
from longest to shortest until Gandi accepts the record, which handles
both apex and wildcard certificates transparently.