Enforces HTTP/HTTPS schemes in Kube Client

What does this MR do and why?

Validates the api_prefix URL schemes before making any request in KubeClient. The schemes are being added because the schemes keyword in the Gitlab::UrlBlocker will be modified to be mandatory later.

I am using the schemes http https as they are the same ones used in the model Clusters::Platforms::Kubernetes validation.

Related to: #372317 (closed)

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