Add schema validation for rabbitmq.additionalPlugins

[ChunyiLyu]

This closes #82

Summary Of Changes

• Add regex pattern validation to prevent shell injection and malformed input
• Enforce a maximum 100 items for additionalPlugins and maximum 100
  characters length for each item; 100 characters and 100 items are just arbitrary limits that I've chosen. There is less than 40 out of the box plugins that rabbitmq 3.8.3 is shipped with. And the longest default plugin name has 73 characters.

Additional Context

• kubebuilder validation tags 'Pattern' and 'MaxLength' must be specified on string type. In order to have the regex pattern and maximun length validation, I had to alias type 'string' as 'Plugin' to specify validations on items of the list. Related kubebuilder github issue can be found here. There is no current plan to support schema validation on list items from kubebuilder. Type alias is the recommended workaround according to the linked github issue.
• This PR also updated the expected-template-output which is part of the rabbitmq chart tests. The rabbitmq chart tests were broken because of a helm version difference between local and CI image. Our CI image is testing with helm v2, while local installed helm is v3. I've updated our CI image to use helm v3, and also updated the the expected-template-output to work with helm v3.

Local Testing

Have ran unit, integration, and system-tests.

[ChunyiLyu]

@mkuratczyk tagging you since you found the security bug.

[Zerpet]

Looks good 🙌 I deployed locally this code and I can't manage to sneak rabbitmqctl commands after a plugin, it returns a validation error, which is a much better option than silently ignoring.

Also verified the charts tests and the diff returns empty string. My machine is a Linux 🐧 so it should behave as in the pipeline.

[j4mcs]

lgtm