refactor(registry): introduce factory with uniform New constructors

[ivankatliarchuk]

What does it do ?

• registry/registry.go was doing two things — defining the interface and wiring up all sub-packages. Splitting it mirrors the existing provider/factory pattern: the interface lives alone (no sub-package imports, no cycles), each sub-package owns its New(cfg, p), and the factory handles selection. Adding named constants eliminates the scattered raw strings.

• each sub-package exposes New(cfg, p) with the uniform signature, and the old verbose constructor becomes unexported newRegistry.

Motivation

Registry to mirror provider factory #6276
#6269 (comment)

More

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

[coveralls]

Pull Request Test Coverage Report for Build 23140224494

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 111 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.07%) to 78.12%

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Files with Coverage Reduction	New Missed Lines	%
noop/noop.go	12	47.83%
awssd/registry.go	14	70.91%
txt/registry.go	21	91.43%
dynamodb/registry.go	64	83.57%

Totals
Change from base Build 23137617424:	-0.07%
Covered Lines:	16570
Relevant Lines:	21211

---

💛 - Coveralls

[mloiseleur]

/lgtm

[ivankatliarchuk]

/test pull-external-dns-unit-test

[ivankatliarchuk]

/approve

[k8s-ci-robot]

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