feat(iam): Add iam v2 Go client libraries/ samples

[vijaykanthm]

Description

Reference to https://b.corp.google.com/issues/339692211 This PR adds v2 API Go client samples for the Get IAM Policies, Set IAM Policies and Test IAM Permissions.

Note: Before submitting a pull request, please open an issue for discussion if you are not associated with Google.

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.

- [securitycenter/iamv2/get_iam_policies.go:17](https://github.com/vijaykanthm/golang-samples/blob/a63a2bf4fe950d0efad7805ff862c5764435d500/securitycenter/iamv2/get_iam_policies.go#L17), tag `securitycenter_get_iam_policies_v2` - [securitycenter/iamv2/set_iam_policies.go:17](https://github.com/vijaykanthm/golang-samples/blob/a63a2bf4fe950d0efad7805ff862c5764435d500/securitycenter/iamv2/set_iam_policies.go#L17), tag `securitycenter_set_iam_policies_v2` - [securitycenter/iamv2/test_iam_permissions.go:17](https://github.com/vijaykanthm/golang-samples/blob/a63a2bf4fe950d0efad7805ff862c5764435d500/securitycenter/iamv2/test_iam_permissions.go#L17), tag `securitycenter_test_iam_permissions_v2`

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[vijaykanthm]

Hi @vijaykanthm, can you please add region tags to your samples? They should look like this: // [START securitycenter_*]. Let me know if you need additional guidance. Thanks!

Thank you for pointing it out. Added them.