Add WhoisXMLAPI Subdomains Lookup plugin for subdomain enumeration

[kato83]

Summary

Add the WhoisXMLAPI Subdomains Lookup API plugin for subdomain enumeration, ported to the current v5.1.1 plugin API.

Port the WhoisXMLAPI subdomain discovery plugin from v4.

Changes

engine/plugins/api/whoisxmlapi.go (new file)

New plugin implementing the WhoisXMLAPI Subdomains Lookup API:

• Queries https://subdomains.whoisxmlapi.com/api/v1 with the configured API key
• Discovers subdomains for in-scope FQDNs and feeds them back into the engine pipeline
• Follows the same patterns as existing API plugins (SecurityTrails, VirusTotal, etc.):
  • Rate limiting via golang.org/x/time/rate (500ms interval)
  • TTL-based caching with support.AssetMonitoredWithinTTL
  • Scope validation with support.HasSLDInScope and e.Session.Scope().IsAssetInScope
  • API key read from datasource config (datasources.yaml)

engine/plugins/load.go

Register api.NewWhoisXMLAPI in the plugin load list.