Support transparent proxy config in kuma-dp via CLI flags

[bartsmykla]

Description

Summary

Extend kuma-dp to support receiving transparent proxy configuration through CLI flags, as described in the MADR Transparent Proxy ConfigMap Handling Outside the Control Plane.

Background

To remove the control plane’s need to manage workload-specific ConfigMaps, the data plane proxy (kuma-dp) must be able to load and merge transparent proxy configuration on its own using a flexible set of CLI options.

Tasks

• Add support for a new --transparent-proxy-config flag to kuma-dp
• Accept the following input formats:
  • No value (enables default transparent proxy settings)
  • Comma-separated list of file paths
  • A - to read config from STDIN
• Allow the flag to be repeated and merge inputs in the order provided
• Add a --transparent-proxy alias for --transparent-proxy-config
• Ensure correct merging logic (later values override earlier ones)
• Implement basic validation of the resulting configuration
• Add tests for different flag combinations and override behavior
• Update CLI help text and documentation

Acceptance Criteria

• kuma-dp accepts transparent proxy config via --transparent-proxy-config and --transparent-proxy flags
• Supported input formats:
  • No value = enable transparent proxy with default config
  • Comma-separated file paths
  • A - for reading from STDIN
• Values are merged in the order provided across repeated flags
• Later values override earlier ones (e.g. STDIN takes precedence if listed last)
• Alias --transparent-proxy works as a shorthand for default config
• Configuration is validated and loaded correctly into the proxy