kube/kube.libsonnet: add OpenAPI.Require

This allows for the following:

    local oa = kube.OpenAPI,

    vaidation: oa.Validation(oa.Dict {
        foo: oa.Required(oa.String),
        bar: oa.Required(oa.Array(oa.Dict {
            baz: oa.Boolean,
        })),
    }),

No more `oa.String { required:: true }`!

Change-Id: I4ecc5002e83a8a1cfcdf083d425d7decd4cf8871

kube/kube.libsonnet

@@ -45,6 +45,10 @@ kube {
             openAPIV3Schema: obj.render,
         },
 
+        Required(inner):: inner {
+            required:: true,
+        },
+
         Dict:: {
             local dict = self,
             required:: false,