feat(all): auto-regenerate .pb.go files (#507)

This is an auto-generated regeneration of the .pb.go files by
cloud.google.com/go/internal/gapicgen. Once this PR is submitted, genbot will
update the corresponding PR to depend on the newer version of go-genproto, and
assign reviewers. Whilst this or any regen PR is open in go-genproto, genbot
will not create any more regeneration PRs. If all regen PRs are closed,
gapicgen will create a new set of regeneration PRs once per night.

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

- Ensure that CI is passing. 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
genbot to assign reviewers to the google-cloud-go PR.

Corresponding google-cloud-go PR: googleapis/google-cloud-go#3448

Changes:
- feat: added API files, GAPIC configuration and BUILD.bazel for Network Connectivity API

  These files are used for generating client libraries in the future.

  PiperOrigin-RevId: 346818879
  Source-Link: googleapis/googleapis@53eb251

- feat: Add the `UNORDERED_LIST` enum value for `google.api.field_behavior`.

  Also includes minor documentation changes that seem to have been made
  in the interim and never published.

  PiperOrigin-RevId: 346664095
  Source-Link: googleapis/googleapis@468a94a

- feat: Add ListSkus, ListOffers API definitions for Cloud Channel API.

  PiperOrigin-RevId: 346583784
  Source-Link: googleapis/googleapis@a53dc02