Move client configuration commands from `thv config` to `thv client`

[amirejaz]

Currently, the commands for registering and removing clients in Toolhive are located under thv config. To create a more consistent and intuitive command structure, we want to move these commands under thv client, aligning them with other client-related commands such as thv client status and thv client setup.

The current commands will be migrated as follows:

• thv config remove-client [Client name] → thv client remove [Client name]
• thv config register-client [Client name] → thv client add [Client name]
• thv config list-registered-clients -> thv client list-registered