feat: Add wildcard DNS support for DNSEndpoint resources

[angelbarrera92]

Fixes #106

Description

This PR implements wildcard DNS support for DNSEndpoint resources, enabling the CoreDNS CRD plugin to resolve queries using wildcard patterns (e.g., *.example.com).

Previously, the plugin only performed exact hostname matching. With this change, DNSEndpoint resources can now define wildcard patterns that match single-level subdomains, following standard DNS wildcard behavior as defined in RFC 4592.

Changes Made

Core Functionality (common/k8sctrl/ctrl.go)

• Added matchesWildcard() function to implement wildcard pattern matching logic
• Modified getEndpointsByCaseInsensitiveName() to support both exact and wildcard matches
• Implemented precedence logic: exact matches take priority over wildcard matches
• Updated margeLocalDNSEndpoints() to properly handle nil labels

Testing (common/k8sctrl/ctrl_test.go)

• Added comprehensive unit tests for matchesWildcard() function (11 test cases)
• Added integration tests for wildcard DNS endpoint resolution
• Added test to verify exact match precedence over wildcards
• All existing tests continue to pass

Documentation (terratest/example/dnsendpoint_wildcard.yaml)

• Created example YAML demonstrating wildcard DNS usage

Wildcard Behavior

Following standard DNS wildcard rules:

• ✅ *.example.com matches test.example.com
• ✅ *.example.com matches app.example.com
• ❌ *.example.com does NOT match example.com (apex domain)
• ❌ *.example.com does NOT match sub.test.example.com (multi-level subdomain)
• ✅ Case-insensitive matching is preserved
• ✅ Exact matches take precedence over wildcard matches

Example Usage

apiVersion: externaldns.k8s.io/v1alpha1
kind: DNSEndpoint
metadata:
  name: wildcard-apps
  labels:
    k8gb.absa.oss/dnstype: local
spec:
  endpoints:
  - dnsName: "*.apps.example.com"
    recordType: A
    targets:
    - 192.168.1.100
    - 192.168.1.101

Now queries for app1.apps.example.com, app2.apps.example.com, etc. will all resolve to the specified targets.

Testing

All tests pass:

go test ./... -short

Specific wildcard tests:

go test ./common/k8sctrl/... -v -run TestWildcard
go test ./common/k8sctrl/... -v -run TestMatchesWildcard

Breaking Changes

None. This is a backwards-compatible feature addition. Existing DNSEndpoint resources with exact hostnames continue to work exactly as before.

Additional Notes

• Wildcard matching follows DNS standards (RFC 4592)
• The implementation is efficient and doesn't impact exact match performance
• Works seamlessly with existing features (GeoIP, round-robin, etc.)

[ytsarev]

@angelbarrera92 thanks for the contribution! Have you tested this change end-to-end?