Add Terraform support for custom versions

[kasey-l]

Custom versions were added to the ReleaseChannel config in the google container API. This change adds it correspondingly to Terraform.

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container: added `custom_versions` field to `google_container_engine_versions` resource

[github-actions]

Googlers: For automatic test runs see go/terraform-auto-test-runs.

@c2thorn, a repository maintainer, has been assigned to review your changes. If you have not received review feedback within 2 business days, please leave a comment on this PR asking them to take a look.

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[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes for commit 59a5c27:

Diff report

Your PR generated the following diffs in downstream repositories:

Repository	Diff Link	Changes
google provider	View Diff	2 files changed, 20 insertions(+)
google-beta provider	View Diff	2 files changed, 20 insertions(+)

Test report

Analytics

Total Tests	Passed	Skipped	Affected
321	306	15	0

Affected Service Packages

• container

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@kasey-l, @c2thorn VCR tests complete for 59a5c27!

[c2thorn]

We’re currently requesting a change in process for Googlers (or individuals on behalf of Googlers) contributing changes to the Terraform Provider for Google Cloud. Please see go/terraform-ssp-adjustment for details.