encoding/openapi: introduce support validator methods
encoding/openapi: introduce support validator methods

ensure consistent error messages and reduce
boilerplate

Issue #56

Change-Id: Ideea2b82ba2d803a0601574c916532b4065097fd
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/2723
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue