Automatic merge from submit-queue

Don't check in zz_generated.openapi.go.

`zz_generated.openapi.go` is the file that causes the most merge conflicts of all. In #33440, @thockin updated the makefile to support generating these files on demand, but that didn't play well with bazel/gazel.

In this PR, I add a new build macro that will generate this file with a `go_genrule`. I added support for keeping the BUILD file up to date in mikedanese/gazel#34.

**Release note**:
```release-note
NONE
```