etcdctl: integrate diagnosis command

This commit completes the migration of the diagnosis tool by integrating it as a new `diagnosis` subcommand in `etcdctl`.

The changes include:
- Adding the command to the main `etcdctl` application.
- Updating Go modules (`go.mod`, `go.sum`) with any new dependencies.
- Documenting the new command and its usage in the `etcdctl` README.md.

This makes the powerful troubleshooting capabilities of the original tool easily accessible to all etcd users.

Resolves #20275

Signed-off-by: Yerasala Venkata, Seshachalam <[email protected]>

Author: seshachalam-yv