fix(cluster-api): Use registryHost for registry mirror credential host

[peirulu]

Add RegistryMirrorCredentialHost field and registryHost helper to strip paths from registry endpoints when configuring container-registry credentials in Bottlerocket. This ensures the credential registry field uses only host:port rather than the full endpoint path.

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Test name	Commit	Details	Required	Rerun command
cluster-api-tooling-presubmit	f4c3aee	link	true	/test cluster-api-tooling-presubmit

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