[cloudbuild] Add invertRegex for GitHub filters

[Maks3w]

https://cloud.google.com/cloud-build/docs/api/reference/rest/v1/projects.triggers#pullrequestfilter
https://cloud.google.com/cloud-build/docs/api/reference/rest/v1/projects.triggers#PushFilter

Release Note Template for Downstream PRs (will be copied)

cloudbuild: Added `invert_regex` for GitHub PushFilter and PullRequestFilter triggers

[modular-magician]

Hello! I am a robot who works on Magic Modules PRs.

I have detected that you are a community contributor, so your PR will be assigned to someone with a commit-bit on this repo for initial review.

Thanks for your contribution! A human will be with you soon.

@rileykarson, please review this PR or find an appropriate assignee.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 2 files changed, 52 insertions(+))
Inspec: Diff ( 3 files changed, 10 insertions(+))

[modular-magician]

Oops! It looks like you're using an unknown release-note type in your changelog entries:

• release-note:enhancement

Please only use the types listed in https://github.com/GoogleCloudPlatform/magic-modules/blob/master/.ci/RELEASE_NOTES_GUIDE.md.

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform Beta: Diff ( 2 files changed, 52 insertions(+))
Inspec: Diff ( 3 files changed, 10 insertions(+))

[rileykarson]

Generally looks good- mind adding the field to a test? https://github.com/GoogleCloudPlatform/magic-modules/blob/master/third_party/terraform/tests/resource_cloudbuild_trigger_test.go#L105

If you'd like, you can run the test locally by generating the output (https://github.com/GoogleCloudPlatform/magic-modules#generating-downstream-tools) and then following https://github.com/terraform-providers/terraform-provider-google/blob/master/.github/CONTRIBUTING.md#tests. If you're unable to, I can run them in our CI once the change is pushed.

[danawillow]

Closing, was added in #3628