Resolve domain names in NAT1To1IP and MediaNAT1To1IP config

[jaylim95]

Previously, these fields only accepted literal IP addresses via netip.ParseAddr. Added support for DNS resolution when SIP LB does not have a static IP but has a stable domain name.

Added resolveAddrOrHost helper that first tries parsing as a literal IP, then falls back to DNS resolution via net.LookupHost, returning the first IPv4 result.

[CLAassistant]

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[codecov]

Codecov Report

❌ Patch coverage is 0% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.72%. Comparing base (0460b40) to head (5b70e0e).
⚠️ Report is 228 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/sip/config.go	0.00%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #595      +/-   ##
==========================================
- Coverage   65.25%   62.72%   -2.53%     
==========================================
  Files          51       35      -16     
  Lines        6588     6909     +321     
==========================================
+ Hits         4299     4334      +35     
- Misses       1915     2152     +237     
- Partials      374      423      +49     

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[dennwc]

Thank you! LGTM after CLA is signed!