Proposal: subcommand system

As [we are planning to integrate more functionalities into this utility]( https://github.com/GreptimeTeam/greptimedb/issues/454). I'm considering to create a subcommand system ahead of time for future extensibility. The idea is to have three categories of commands:

- `gtctl cluster` manages cluster on kubernetes or other platform. We will need to migrate all existing commands into this category, say `gtctl create cluster` is to be renamed to `gtctl cluster create`
- `gtctl db` manages databases
- `gtctl cloud` manages cloud accounts

Note that this architecture is from user's perspective. Internally we don't need to integrate all functionalities into one binary and can utilize an architecture like `git` or `cargo` to forward calls to certain binary. For example, `gtctl db` to `gtctl-db`

The issue names no files or tests. Start by locating the current gtctl command entry point and the existing create cluster command, then trace how commands are dispatched. Done would require an agreed subcommand architecture and migration plan for cluster, db, and cloud commands.

Written by the indexing model from the issue text.