feat: regional template operations samples

[BigBlackWolf]

Description

Created basic operations for regional templates

Checklist

• [ ] I have followed Contributing Guidelines from CONTRIBUTING.MD
• [ ] Tests pass: go test -v ./.. (see Testing)
• [ ] Code formatted: gofmt (see Formatting)
• [ ] Vetting pass: go vet (see Formatting)
• [ ] These samples need a new API enabled in testing projects to pass (let us know which ones)
• [ ] These samples need a new/updated env vars in testing projects set to pass (let us know which ones)
• [ ] This sample adds a new sample directory, and I updated the CODEOWNERS file with the codeowners for this sample
• [ ] This sample adds a new Product API, and I updated the Blunderbuss issue/PR auto-assigner with the codeowners for this sample
• [ ] Please merge this PR for me once it is approved

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 3 region tags.

- [compute/instance-templates/create-instance-templates/create_regional_template.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/86726c0f4e2a47eb273d62de881b6d4ac06a0cbc/compute/instance-templates/create-instance-templates/create_regional_template.go#L17), tag `compute_regional_template_create` - [compute/instance-templates/create-instance-templates/delete_regional_template.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/86726c0f4e2a47eb273d62de881b6d4ac06a0cbc/compute/instance-templates/create-instance-templates/delete_regional_template.go#L17), tag `compute_regional_template_delete` - [compute/instance-templates/create-instance-templates/get_regional_template.go:17](https://github.com/GoogleCloudPlatform/golang-samples/blob/86726c0f4e2a47eb273d62de881b6d4ac06a0cbc/compute/instance-templates/create-instance-templates/get_regional_template.go#L17), tag `compute_regional_template_get`

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