Proposal: subcommand system

[sunng87]

As we are planning to integrate more functionalities into this utility. 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