feat(commands): Introduce subcommand as a first-class command abstraction

[geetanshjuneja]

Description

This PR adds a subcommand registration framework as well resolution of command to the correct subcommand. This PR contains all the changes of #3413 while addressing some reviewer comments in it.
Following changes are included in this PR:

• MakeSubCmdAttr() is used to register subcommand metadata, such as arity, flags, key specs, and category.
• Command registration flow now also register the subcommand in a dedicated map which is used to lookup subcommand during command resolution.
• Updated command creation paths in client, lua command invocation, command get keys.
• Added a reference implementation for NAMESPACE subcommands without changing its existing behaviour.
• Added C++ units tests for root command resolution, registered subcommand resolution, unknown subcommand fallback, arity checks, key range extraction, and renamed root command behavior.
• Added Go integration tests for command getkeys, namespace root command and subcommands.

AI disclosure
Used AI to add namspace subcommand integration tests in tests/gocase/unit/namespace/namespace_test.go.

[jihuayu]

Hello @geetanshjuneja. Thanks for your contribution. If you used an LLM, please follow our guidelines for AI-assisted contributions: https://kvrocks.apache.org/community/contributing#guidelines-for-ai-assisted-contributions
This does not mean that using an LLM is prohibited. It simply helps us assess the reliability of the code.
I would like the PR description to reflect your own understanding and reasoning, rather than being filled with LLM-generated content.

[geetanshjuneja]

Hi @jihuayu, updated the description. Changes in this PR are exactly same as your PR, once this is merged I can continue refactoring other subcommands.