provider: add credential source boundary

[yuminn-k]

What

• Add a provider credential source abstraction with system-env and map-backed implementations.
• Add SelectWithCredentialSource at the provider factory boundary so future per-pipeline config can construct providers without mutating process env.
• Migrate Cloudflare, DNSimple, NS1, Civo, Gandi, and Linode provider construction to read credentials from the injected source while keeping existing env-backed behavior for current callers.
• Add tests proving explicit sources do not fall back to global env and same-type providers can be constructed with distinct credentials.

Why

Follow-up slice from #6512 for the multi-provider CRD design in #1961. This establishes the credential boundary needed before wiring credentialsRef into pipeline construction.

Notes

This does not wire CRD credentialsRef yet. OCI workload identity, CoreDNS etcd env, and Scaleway SDK env handling are left as provider-specific follow-ups because they need different treatment than direct credential env reads.

Validation

• go test ./provider/credentials ./provider/factory ./provider/ns1 ./provider/civo ./provider/linode ./provider/gandi ./provider/cloudflare ./provider/dnsimple
• go test ./provider/...
• go test ./...
• PATH="$HOME/go/bin:$PATH" make lint

[kubernetes-prow]

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[kubernetes-prow]

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[kubernetes-prow]

Hi @yuminn-k. Thanks for your PR.

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[ivankatliarchuk]

Too early. I'm not sure about the approach