feat: add logs subcommand

[loheagn]

This PR adds the logs subcommand.

The usage of this subcommand is:

$ ofn logs -h
Get the logs from the serving pods created by the function

Usage:
  ofn logs [OPTIONS] FUNCTION_NAME [CONTAINER_NAME]

Examples:

  # Get tht logs from the 'function' container in the serving pods created by the function whose name is 'demo-function'
  ofn logs demo-function

  # Get tht logs from the 'extra' container (a container whose name is not 'function') in the serving pods created by the function whose name is 'demo-function'
  ofn logs demo-function extra

  # Begin streaming the logs from the 'function' container in the serving pods created by the function whose name is 'demo-function'
  ofn logs -f demo-function


Flags:
  -f, --follow   Specify if the logs should be streamed
  -h, --help     help for logs

Global Flags:
      --as string                      Username to impersonate for the operation
      --as-group stringArray           Group to impersonate for the operation, this flag can be repeated to specify multiple groups.
      --cache-dir string               Default cache directory (default "/Users/loheagn/.kube/cache")
      --certificate-authority string   Path to a cert file for the certificate authority
      --client-certificate string      Path to a client certificate file for TLS
      --client-key string              Path to a client key file for TLS
      --cluster string                 The name of the kubeconfig cluster to use
      --context string                 The name of the kubeconfig context to use
      --insecure-skip-tls-verify       If true, the server's certificate will not be checked for validity. This will make your HTTPS connections insecure
      --kubeconfig string              Path to the kubeconfig file to use for CLI requests.
  -n, --namespace string               If present, the namespace scope for this CLI request
      --password string                Password for basic authentication to the API server
      --request-timeout string         The length of time to wait before giving up on a single server request. Non-zero values should contain a corresponding time unit (e.g. 1s, 2m, 3h). A value of zero means don't timeout requests. (default "0")
  -s, --server string                  The address and port of the Kubernetes API server
      --tls-server-name string         Server name to use for server certificate validation. If it is not provided, the hostname used to contact the server is used
      --token string                   Bearer token for authentication to the API server
      --user string                    The name of the kubeconfig user to use
      --username string                Username for basic authentication to the API server

Fix #36

Signed-off-by: loheagn loheagn@icloud.com

[CLAassistant]

All committers have signed the CLA.

[tpiperatgod]

lgtm, @benjaminhuo @wentevill can you help review this?

[wentevill]

lgtm

[benjaminhuo]

Thanks @loheagn, This is a pretty good PR!