Feature Request: WAN IP discovery for Let's Encrypt IP cert

I implemented ACME WAN IP discovery for Let's Encrypt IP certs

I've only tested this using Hysteria2, there may be bugs with other protocols

The idea is to add optional WAN-IP discovery so ACME IP certificates do not require
rewriting "domain": [], when the public address changes.

• lookup runs at startup (start fails if the first lookup fails)
• interval time to check and re-issue only when the address set changes
• forced tcp4/tcp6 dial so dual-stack echo endpoints return the matching family
• GetCertificate selects IPv4 vs IPv6 by SNI, local addr, then remote family
  (fixes empty/"::" SNI on listen "::")
• deletes stale IP cert files under "data_directory":

It probably needs some more beautifying in code and json structure

new json parameters in acme type

  "certificate_providers": [
    {
      "type": "acme",
      "tag": "",
      "provider": "letsencrypt",
      "check_ip": true,
      "check_ip_interval": "2d",
      "check_ip_url": "https://api.ip.sb/geoip",
      "check_ip_version": "both"

• check_ip: Enable WAN-IP discovery. domain may be empty. Static names in domain are still requested.
• check_ip_url: Required if check_ip is true. Plain text or JSON (ip, query, origin, address, wan_ip, ipAddress, IPv4, ipv6).
• check_ip_version: 4 or empty → IPv4 only; 6 → IPv6 only; both → forced tcp4 and tcp6 lookups, one cert per address.
• check_ip_interval: Default/omitted/empty lets certmagic handle the renewal process using the shortlived schedule, Accepts durations such as 30m, 2h, 5d

Branch on my fork:
https://github.com/FreestyleSasuke/sing-box/tree/feat/acme-check-ip

Compare:
https://github.com/SagerNet/sing-box/compare/testing...FreestyleSasuke:feat/acme-check-ip

Start by reviewing the provided feat/acme-check-ip branch and its comparison with testing, then inspect how the ACME certificate provider currently handles domains and certificate files. Check the proposed JSON parameters and the stated startup, interval, address-family, certificate-selection, and stale-file behaviors. Done means the WAN address changes are handled without rewriting the domain list and the documented configuration works across the supported protocols.