Add ability to pass extra arguments to kustomize build command

[Copilot]

This PR adds support for passing extra arguments to the kustomize build command, which is needed for kustomize plugins like ksops that require additional flags such as --enable-exec.

Changes Made

Core Implementation

• Added ExtraArgs []string field to KustomizeBuildOpts struct - allows passing extra arguments to kustomize build operations
• Added KustomizeBuildArgs []string field to ChartifyOpts struct - exposes the functionality at the main API level
• Added ExtraArgs []string field to PatchOpts struct - ensures consistent support across all kustomize operations
• Modified kustomize build command construction - extra arguments are appended to the command after standard flags

CLI Support

• Added --kustomize-build-arg flag - can be specified multiple times to pass extra arguments
• Updated help documentation - includes usage examples for the new flag

Testing

• Added comprehensive unit tests - verify field initialization and argument handling
• Verified end-to-end functionality - tested CLI with real kustomize operations

Usage Examples

Single extra argument

chartify --kustomize-build-arg="--enable-exec" release-name /path/to/kustomization

Multiple extra arguments

chartify --kustomize-build-arg="--enable-exec" --kustomize-build-arg="--verbose" release-name /path/to/kustomization

Programmatic usage

opts := &chartify.ChartifyOpts{
    KustomizeBuildArgs: []string{"--enable-exec", "--some-other-flag"},
}

Implementation Details

The extra arguments are appended to the kustomize build command after all standard flags:

kustomize build --load-restrictor=LoadRestrictionsNone --enable-helm --helm-command=helm --enable-exec /path/to/build

This approach ensures compatibility with existing functionality while providing the flexibility needed for kustomize plugins.

Fixes #113.

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