refactor(endpoint): export ProviderSpecificAlias and consolidate usage

[u-kai]

What does it do ?

Export providerSpecificAlias as ProviderSpecificAlias in the endpoint package and replace all
hardcoded "alias" string literals and duplicated local definitions across the codebase with
the single exported constant.

Motivation

The "alias" key was defined in multiple places (endpoint/endpoint.go, provider/aws/aws.go) and
used as a raw string literal in many others (registry/txt/registry.go, provider/pdns/pdns.go, source/wrappers/post_processor.go, etc.).

By exporting a single constant, all consumers reference one source of truth, and the TODO comment requesting this consolidation is resolved.

More

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

[u-kai]

AliasKey and ProviderSpecificAlias are related in that the annotation value set via AliasKey
is eventually converted into a ProviderSpecific property named ProviderSpecificAlias by the
source/annotations package.

However, they are separate definitions serving different purposes — one is a Kubernetes annotation key with a full prefix, the other is an internal property name — so keeping them as independent constants is the right approach.

The original TODO has been removed accordingly.

[coveralls]

Pull Request Test Coverage Report for Build 23925973436

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 46 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-0.004%) to 80.524%

---

Files with Coverage Reduction	New Missed Lines	%
openshift_route.go	1	82.93%
aws/aws.go	45	89.22%

Totals
Change from base Build 23903893422:	-0.004%
Covered Lines:	17158
Relevant Lines:	21308

---

💛 - Coveralls

[ivankatliarchuk]

/approve

[k8s-ci-robot]

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