feat(extension-api): allow to specify enum type for configuration items

[benoitf]

What does this PR do?

extension/internal components can provide enum type for properties

Screenshot/screencast of this PR

KegoYh8ZqE.mp4

What issues does this PR fix or reference?

Related to #413

How to test this PR?

Add into an extension enum type

example for podman extension in package.json/contributes

        "podman.factory.example": {
          "type": "string",
          "default": "foo",
          "scope": "ContainerProviderConnectionFactory",
          "description": "Name",
          "enum": ["foo", "bar", "baz"]
        },

Change-Id: I69b2c00642cc9b1162add7090e7f95974ba143f2
Signed-off-by: Florent Benoit fbenoit@redhat.com

[cdrage]

LGTM and see no errors, tested it out using contributes and it works fine!