Find a better naming for chePlugin spec reference

[l0rd]

Currently the following syntax are used to reference the spec in a che-plugin registry or in a yaml file published on the internet respectively:

components:
  - chePlugin:
      registry:
        id: redhat/vscode-yaml/latest
        registryUrl: "external-registry-url"
  - chePlugin:
      url: https://gist.../meta.yaml

That may be confusing. We should find a better name.

[davidfestal]

In the PR #29, that introduces changes agreed on in issue #25, I included the following proposal which would also be useful to reference a parent devfile inside a devfile registry:

components:
  - chePlugin:
      registryEntry:
        id: redhat/vscode-yaml/latest
        baseUrl: "external-registry-url"
  - chePlugin:
      url: https://gist.../meta.yaml

To be fully consistent with the parent case, we could use uri instead of url when pointing directly to a plugin meta.yaml

[davidfestal]

Implemented