add support for transformer goplugins

[Liujingfang1]

1. Load the file content from paths listed under transformers as resources.
2. Read the Kind from the resource and treat it as the goplugin name
3. The goplugin is searched under $XDG_CONFIG_HOME/kustomize/plugins, if $XDG_CONFIG_HOME is not defined, search $HOME/.config/kustomize/plugins
4. The goplugin is configued (you can also think of this as input arguments to a program) by the resource loaded in step 1.
5. Also need to run kustomize build -enable_alpha_goplugins_accept_panic_risk to allow goplugins

[k8s-ci-robot]

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[monopole]

/lgtm