feat: add K8S_CONTEXT configuration option

[m-k8s]

Summary

Add support for configuring Kubernetes context via K8S_CONTEXT setting, similar to the existing K8S_NAMESPACE configuration.

Changes

• Add K8S_CONTEXT configuration option in defaults (empty by default)
• Update all kubectl commands to use --context flag when configured
• Update Kubernetes Python API client to load the specified context
• Add kubectl_base_args() helper function following the DRY principle

Behavior

• When K8S_CONTEXT is empty (default): uses the current context from kubeconfig
• When K8S_CONTEXT is set: all kubectl commands and API calls use the specified context

Testing

Tested with a fake context set as default in kubeconfig to ensure Tutor uses the configured K8S_CONTEXT instead of the kubeconfig's current-context.

[m-k8s]

Waiting for approval to run the "Run tests" workflow before marking this PR as ready for review.

[mlabeeb03]

@m-k8s Thanks for the PR. I am trying to understand the need for this feature considering we already have kubectl config use-context.

[mlabeeb03]

Hey @m-k8s just checking in. If you can provide additional context of why you need it and what problems you are facing with kubectl config use-context then we can move forward with the review.