Support the caFile file in the Kubernetes plugin configuration

[jpeach]

Feature Suggestion

As noted in https://github.com/backstage/backstage/pull/12911, the Kubernetes cluster caData field needs to be base64 encoded.

However, it looks like the Kubernetes client also supports a caFile field that reads the raw value from a file. This would work really well when Backstage is getting CA certificated from Kubernetes secrets volumes.

Possible Implementation

I image that everywhere the plugin passes caData down to the Kubernetes client, it also needs to pass caFile.

Context

Deploy Backstage in a Kubernetes cluster. The Kubernetes plugin is configured to scan multiple clusters. The CA certificates for each cluster are available as secrets in the deployment namespace.

[jpeach]

ref https://github.com/backstage/backstage/issues/13762

[freben]

ping @mclarke47 here

[github-actions[bot]]

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