PR #60 follow-up fixes

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Follow-up to #60.

• FileCredentials ignores unknown JSON keys, so the AWS Secrets Manager format with engine, host, port and dbname works. The test whenFileHasExtraKeys_shouldIgnoreThem covers it.
• KubernetesServiceTest.java and FileRef.java converted from CRLF to LF, like the rest of the repo.
• Member order in KubernetesService and the two test classes, the Jackson import position, and constructor injection for the two config values in ClusterConnectionReconcilerTest.
• CEL rule simplified to has(self.adminSecretRef) != has(self.adminSecretFileRef). Same truth table, shorter.
• #### File format as a child of ### FileRef in the docs, Javadoc wording fixes, and var for the AtomicReference.

Not in this PR, they come separately: Helm chart values for volumes and volumeMounts, a .gitattributes that forces LF, and the Error Prone Varifier check.

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@fredordercloud as the line feed changed for two files, you have to enable Hide whitespace in the PR file diff to see the real changes.