feat: Add ping command to check Harbor connectivity

[ravjot07]

Description:
This PR introduces a new ping command to the Harbor CLI, allowing users to verify connectivity with the Harbor server. The command calls the existing api.Ping() function and returns a success message (Pong!) if the server is reachable.

Changes Introduced:

• Added a new ping command in cmd/harbor/root/ping.go.
• Registered PingCommand() in cmd/harbor/root/cmd.go.
• Calls api.Ping() from pkg/api/ping_handler.go.

Usage Example:

harbor ping

Output (if successful):

Pong! Successfully contacted the Harbor server.

contributes towards #94 and #315

[Standing-Man]

Hi @ravjot07, thank you for your contribution! The ping handler has already been implemented in PR #285. If you're still interested in this project, feel free to explore other issues to work on.