pkg/list: add list builtins for OpenAPI features

MaxItems, MinItems, UniqueItems, and Contains

Also added conversion to OpenAPI (except for
Contains, which is no supported by the latter).

Change-Id: I2edf4e8572acc08e8ced4a61918dff091325f3a0
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2642
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/encoding/openapi/testdata/array.cue b/encoding/openapi/testdata/array.cue
index 3261f81..9f28a52 100644
--- a/encoding/openapi/testdata/array.cue
+++ b/encoding/openapi/testdata/array.cue
@@ -1,6 +1,12 @@
+import "list"
+
 Arrays: {
 	bar?: [...MyEnum]
 	foo?: [...MyStruct]
+
+	baz?: list.UniqueItems()
+
+	qux?: list.MinItems(1) & list.MaxItems(3)
 }
 
 Arrays: {