GIDINET: Auto-suggest nameservers and add premium DNS helper

[zupolgec]

Summary

• GetNameservers now returns the static free-tier Gidinet nameservers (dnsl1/dnsl2.gidinet.com). Previously it returned nil, nil, which meant DNSControl could not derive registrar delegation from the DNS provider for Gidinet-hosted zones. With this change, zones declared as D(..., REG_GIDINET, DnsProvider(DSP_GIDINET), ...) get correct NS suggestions automatically, matching the behavior of providers like Cloudflare or netcup.
• New GIDINET_PREMIUM_NS() JS helper that emits NAMESERVER("dns1..dns5.gidinet.com.") for zones on Gidinet's premium DNS tier. Intended to be paired with DnsProvider(DSP_GIDINET, 0) so the free-tier defaults from GetNameservers are skipped and only the five premium nameservers drive the delegation.
• filterApexNS now drops apex NS records silently when they were synthesized from dc.Nameservers (either auto-injected by GetNameservers or declared via NAMESERVER()), since AddNSRecords adds those to dc.Records automatically. Previously it warned on every preview once GetNameservers began returning values. Truly unexpected apex NS targets (e.g. a stray NS("@", "something-else.")) still emit a warning because the Gidinet DNS API cannot honor them.
• Documentation updated to describe both tiers and the GIDINET_PREMIUM_NS() usage pattern.

Motivation

Before this change, a zone served by Gidinet DNS required a redundant NS_GIDINET()-style block of explicit NAMESERVER() calls just so REG_GIDINET could propose the correct delegation. Users hit a no-op/warning loop where the registrar had nothing to compare against and/or where apex NS records raised spurious warnings. Gidinet's nameservers are static per-tier, so returning them from GetNameservers is safe and matches the convention used by other providers.

Test plan

• go test ./...
• go vet ./...
• gofmt -l providers/gidinet/ pkg/js/
• dnscontrol check on a real Gidinet-hosted configuration
• dnscontrol preview on a free-tier zone: registrar correctly proposes dnsl1/dnsl2.gidinet.com delegation without explicit NAMESERVER() in the zone file; no spurious apex-NS warnings.
• dnscontrol preview on a premium-tier zone using DnsProvider(DSP_GIDINET, 0) + GIDINET_PREMIUM_NS() (pending availability of a premium zone for the author to validate end-to-end).
• IR inspection (print-ir) on a synthetic premium-tier config confirms the helper expands to the five NAMESERVER("dns1..5.gidinet.com.") entries.

Notes

• No interface or API changes.
• Backwards compatible: zones that already include explicit NAMESERVER("dnsl1/dnsl2…") records still work (the registrar-side dedup already handles duplicates).

[TomOnTime]

Thank you for sending this!

Right now the project is in transition but I hope to be able to address this once we've moved to our new git org.