Add UI extensions commands

[nvladimirovi]

Following commads are added: generate, list, deploy, delete. They will help the user to manage his ui extensions.

CLI Spinner class is implemented to indicate the action of the user.

Extension generator is class which inherits generator abstract class and give the ability to copy existing template from the file system, of the user, and populate it with the data, given by the user.

Prompt and PromptLauncher classes are build on top of python click lib to give the ability to validate and collect the data from the user. Signed-off-by: Nikola Vladimirov Iliev [email protected]

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[rocknes]

vcd-cli project is not the right place to craft REST requests or write business logic, pyvcloud is the right place to craft REST reuqest and process the responses. I think this PR needs to be split into two PRs one on pyvcloud dealing with REST + business logic and another one in this project dealing with just the cli aspect of the commands.

[nvladimirovi]

Thank you for your comment, I moved the UiPlugin class to pyvcloud project, but ExtGenerator class has to stay in vcd-cli because this class depends on other classes, which are related to the CLI functionality.