feat(securitymarksv2): Add Security Marks 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 Adding & Deleting Security Marks, Add Security Marks to Findings, Delete Security Marks.

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/securitymarksv2/add_delete_security_marks.go:17](https://github.com/vijaykanthm/golang-samples/blob/d8aecde4c544af01540dd209b9380d8883547cb2/securitycenter/securitymarksv2/add_delete_security_marks.go#L17), tag `securitycenter_add_delete_security_marks_v2` - [securitycenter/securitymarksv2/add_security_marks.go:17](https://github.com/vijaykanthm/golang-samples/blob/d8aecde4c544af01540dd209b9380d8883547cb2/securitycenter/securitymarksv2/add_security_marks.go#L17), tag `securitycenter_add_security_marks_v2` - [securitycenter/securitymarksv2/delete_security_marks.go:17](https://github.com/vijaykanthm/golang-samples/blob/d8aecde4c544af01540dd209b9380d8883547cb2/securitycenter/securitymarksv2/delete_security_marks.go#L17), tag `securitycenter_delete_security_marks_v2`

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