ddns-scripts: add API-based registered IP verification for Cloudflare proxied

ddns-scripts: add API-based registered IP verification for Cloudflare proxied
records

Problem:
When using Cloudflare with proxy enabled (orange cloud), DNS lookups
return Cloudflare's edge IP instead of the actual origin IP registered
in the dashboard. This causes ddns-scripts to incorrectly detect IP
mismatches, triggering unnecessary updates and potential rate limiting.

Solution:
Add an optional 'use_api_check' configuration option that enables
provider scripts to fetch the registered IP directly via their API,
bypassing DNS lookups.

Changes:
- dynamic_dns_functions.sh: Add API check block to get_registered_ip()
  (~25 lines). When use_api_check is enabled, sources the provider
  script with GET_REGISTERED_IP=1 flag. Falls back to DNS lookup if
  API check is disabled, unsupported, or fails.

- update_cloudflare_com_v4.sh: Add handler for GET_REGISTERED_IP mode
  (~15 lines). Reuses existing cURL setup and authentication to query
  Cloudflare API for actual record content.

- etc/config/ddns: Document use_api_check option

Behavior:
- use_api_check=0 or unset: DNS lookup (existing behavior, no changes)
- use_api_check=1 with API support: API query for registered IP
- use_api_check=1 without API support: Falls back to DNS lookup
- API failure: Gracefully falls back to DNS lookup

Testing:
- Cloudflare (proxied): Correctly retrieves origin IP via API
- Cloudflare (non-proxied): Works correctly
- No-IP: DNS lookup works (no regression)
- IPv4 and IPv6 records tested
- API failure gracefully falls back to DNS

Signed-off-by: Wayne King 244781262+bdk38@users.noreply.github.com