auto-regenerate .pb.go files (#268)

This is an auto-generated regeneration of the .pb.go files by autogogen. Once
this PR is submitted, autotogen will update the corresponding CL at gocloud
to depend on the newer version of go-genproto, and assign reviewers. Whilst this
or any regen PR is open in go-genproto, autogogen will not create any more
regeneration PRs or CLs. If all regen PRs are closed, autogogen will create a
new set of regeneration PRs and CLs once per night.

If you have been assigned to review this CL, please:

- Ensure that CI is passin If it's failing, it requires your manual attention.
- Approve and submit this PR if you believe it's ready to ship. That will prompt
  autogogen to assign reviewers to the gocloud CL.

Corresponding gocloud CL: https://code-review.googlesource.com/c/gocloud/+/48479