encoding/openapi: date-time & string format
encoding/openapi: date-time & string format

There is no ```string``` format and the date-format in OpenAPI is ```date-time```.
https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.0.md#data-types

Closes #426
https://github.com/cuelang/cue/pull/426

GitOrigin-RevId: f8270d4c331537d23eb41a936d663dff84ad4d18
Change-Id: I5b09ace4d4ca9170df368710fea135da68f38b37
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/6420
Reviewed-by: Marcel van Lohuizen <a22d082192f3ccf984644a4f482781f4c2d65180@golang.org>

cuelang/cue