feat(source/gateway-api): add Gateway resource source

[danicaceres1998]

What does it do ?

Unlike the route sources (gateway-httproute etc.) which resolve hostnames by traversing Route → parentRef → Gateway, this source targets the Gateway object itself — useful when DNS is managed at the infrastructure level rather than per-route.

• Adds a new gateway source type (--source=gateway) that watches Gateway resources directly and publishes DNS records (A/AAAA/CNAME) from status.addresses
• This source targets the Gateway object itself via the external-dns.alpha.kubernetes.io/hostname annotation
• Adds 16 unit tests covering all filtering, record type selection, and annotation behaviors
• Updates docs: flags.md, sources/index.md (regenerated), and sources/gateway-api.md

Supported features:

• Hostname annotation (external-dns.alpha.kubernetes.io/hostname)
• Target override annotation (external-dns.alpha.kubernetes.io/target)
• TTL annotation, provider-specific annotations
• GatewayName / GatewayNamespace / GatewayLabelFilter / AnnotationFilter
• FQDN template (--fqdn-template flag)
• Controller annotation mismatch filtering

Motivation

Users managing DNS at the infrastructure level (one Gateway = one external IP) should not need to annotate every individual Route. This source allows ExternalDNS to watch Gateways directly.

I'm working on a Gateway-as-a-Service (GaaS) architecture. The goal is to only manage headless Gateways and rely on the HTTPRoute resources to manage redirections within the cluster. When I was performing some tests and validations, I just found that the project does not support the native Gateway source, so decided to carry out the implementation.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: danicaceres1998 / name: Juan Ojeda (8f7e69c)

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[danicaceres1998]

/easycla

[danicaceres1998]

/easycla