Add --only-new flag to optimize kubeconfig updates

[Copilot]

Fixes the issue where the update command would always export all EKS clusters to kubeconfig, even if they were already present. This resulted in unnecessary updates and potential kubeconfig bloat.

Changes

This PR adds a new --only-new flag to the aws update command that enables selective cluster export. When enabled, the command will:

• Check each discovered cluster against the existing kubeconfig using the existing IsExported() method
• Skip clusters that are already exported (same endpoint already exists in a context)
• Only add truly new clusters to the kubeconfig
• Provide informative output showing the count of new vs skipped clusters

Usage

# Default behavior (unchanged) - exports all clusters
kdiscover aws update

# New optimized behavior - only exports new clusters
kdiscover aws update --only-new

When using --only-new, the output will show:

Found 4 clusters remote
Added 2 new clusters, skipped 2 already exported clusters

Implementation Details

The implementation leverages the existing kubeconfig.IsExported() method which checks if a cluster with the same endpoint already exists in a context. This maintains consistency with the existing "exported locally" logic used in the list command.

The change is backward compatible - the default behavior remains unchanged, and the new flag is opt-in.

Testing

• Added comprehensive test Test_onlyNewFlagLogic that validates the filtering behavior
• All existing tests continue to pass
• Manual verification of CLI help and flag functionality

Closes #[issue_number]

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Optimize kubeconfig update</issue_title>
<issue_description>Currently, we update the configuration for each cluster found, Even if there is already a cluster with the same identity. Provide a flag to allow exporting only the currently unexported clusters. </issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #43

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