Add skeleton of the standalone-cluster CLI plugin

[joshrosso]

What this PR does / why we need it:

This PR adds the skeleton of the standalone-cluster plugin. It does
not implement any functionality. Instead, it creates the directories and
files needed to begin the implementation.

I've also added the necessary libraries to utilize tkg-cli.

Which issue(s) this PR fixes:
Resolves: #386

Describe testing done for PR:

To validate this PR, I have run:

make check
make install-cli-plugins

I have also verified the plugin shows up in the root help text.

�[1mTanzu CLI�[0m

�[1mUsage:�[0m
  tanzu [command]

�[1mAvailable command groups:�[0m

  �[1mAdmin�[0m
    builder                 Build Tanzu components 
	
  �[1mManage�[0m
    package                 Package management 
	
  �[1mRun�[0m
    cluster                 Kubernetes cluster operations 
    kubernetes-release      Kubernetes release operations 
    management-cluster      Kubernetes management cluster operations 
    standalone-cluster      Create clusters without a dedicated management cluster 
	
  �[1mSystem�[0m
    completion              Output shell completion code 
    config                  Configuration for the CLI 
    init                    Initialize the CLI 
    login                   Login to the platform 
    plugin                  Manage CLI plugins 
    update                  Update the CLI 
    version                 Version information 
	
  �[1mVersion�[0m
    alpha                   Alpha CLI commands 
	

�[1mFlags:�[0m
  -h, --help   help for tanzu

Use "tanzu [command] --help" for more information about a command. 

Not logged in

And that the plugin itself can be called.

Create clusters without a dedicated management cluster

�[1mUsage:�[0m
  tanzu standalone-cluster [command]

�[1mAvailable Commands:�[0m
  create      create a standalone workload cluster

�[1mFlags:�[0m
  -h, --help   help for standalone-cluster

Use "standalone-cluster [command] --help" for more information about a command.

Special notes for your reviewer:
Please review this PR and verify the above.

Does this PR introduce a user-facing change?:

introduce standalone-cluster CLI plugin

[joshrosso]

@dvonthenen this is ready for review.

There are a few important implications of this PR.

1. It brings all k8s-realated libraries into a consistent version and makes them consistent with core and tkg-cli.
2. In order to keep these versions consistent, yet no run into the OpenAPIV2 package bug, it creates forks of some repositories.

To ensure we don't lose track of simplifying go.mod in the future, I've added #413