feat: Migrate CLI from Kingpin to Cobra for better autocompletion and maintainability

[joshichintan]

Rationale

• Kingpin’s autocompletion support is limited and difficult to extend.
• Cobra provides robust, built-in support for Bash, Zsh, Fish, and PowerShell completions.
• Cobra is more actively maintained and widely adopted (used by tools like kubectl, helm, gh, etc.), aligning us with modern CLI standards.

Benefits

• Greatly improved shell autocompletion (dynamic flags and subcommands)
• Clearer subcommand structure and help text
• Easier long-term maintenance and community contribution

Kingpin has served well, but its autocompletion model has become a bottleneck for usability.
This migration modernizes the CLI foundation and should make future enhancements simpler and more reliable.

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[mbevc1]

Hi @joshichintan - thanks for your PR! Seems there are still some wrinkles we'd need to iron out and I've left a comment here: https://github.com/ByteNess/aws-vault/pull/172/changes#r2624625875

Your changes are merged to v8 branch.