Merge meta.yaml and plugin yaml in v3

[garagatyi]

What does this PR do?

Merge meta.yaml and plugin yaml and put it to v3 folder as described here.
One thing that I changed comparing to the comment linked above is that now both theia and vscode plugins are using field plugins to refer to the archives of the plugins.
Example of theia plugin:

....
spec:
  plugins:
    - /~https://github.com/redhat-developer/omnisharp-theia-plugin/releases/download/v0.0.1/omnisharp_theia_plugin.theia

And example of vscode plugin:

...
spec:
  plugins:
    - /~https://github.com/Azure/vscode-kubernetes-tools/releases/download/0.1.17/vscode-kubernetes-tools-0.1.17.vsix

@l0rd how do you think is it OK to refer to archives as plugins in both of them? Should we change to extensions for vscode and plugins for theia or should we use something completely different, like archives?

@tsmaeder FYI you can see in this PR how it will be possible to configure env vars, memory, volumes, etc for vscode extension in the registry. In this PR, in particular, VSCode extensions metas contain only container image, but since foramt is absolutely the same as with editors you can see how to do it in the theia meta, for example.

[l0rd]

Most of the files embedded in containers are vs code extensions (not theia plugin) and I would expect this to become more and more the case (do we really need theia plugins after all? isn't it just confusing to support both? plugins may be replaced by vs code extensions in the future). Hence extensions would be better than plugins. Otherwise you can use references or even just files.

[garagatyi]

Makes sense. I'll go with field name extensions

[garagatyi]

@l0rd changed plugins to extensions

[garagatyi]

@l0rd @ibuziuk please, review

[ibuziuk]

LGTM, the only thing that I'm a bit baffled with is that v3 plugins have v2 API version. Should not those 2 be consistent to avoid confusion? I thought that:
v1 - first version of plugins we had
v2 - naming convention changes
v3 - new format of meta.yaml (this PR)

[garagatyi]

@ibuziuk we have different folders for different versions of the registry, but this doesn't mean those versions use different format of meta.yaml.
The reason I put v2 as apiVersion is that it is second format we have that is not compatible with the current one, which i supposed to be v1. Naming convention didn't change format of the meta.yaml file, but did change placement of files.