feat(spanner): add samples for using multi-region encryption keys for backups and database

[rahul2393]

Description

Reference PRs:

• feat[Spanner]: Add Multi Region Encryption samples dotnet-docs-samples#2832
• feat(spanner): add MR CMEK samples php-docs-samples#2044
• docs: allow multiple KMS keys to create CMEK database/backup googleapis/python-spanner#1191
• feat: allow multiple KMS keys to create CMEK database/backup googleapis/nodejs-spanner#2099
• docs: add Multi Region Encryption samples googleapis/java-spanner#3524

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]

Here is the summary of changes.

You are about to add 4 region tags.

• spanner/spanner_snippets/spanner/spanner_copy_backup_with_mr_cmek.go:17, tag spanner_copy_backup_with_MR_CMEK
• spanner/spanner_snippets/spanner/spanner_create_backup_with_multi_region_encryption_key.go:17, tag spanner_create_backup_with_MR_CMEK
• spanner/spanner_snippets/spanner/spanner_create_database_with_multi_region_encryption_key.go:17, tag spanner_create_database_with_MR_CMEK
• spanner/spanner_snippets/spanner/spanner_restore_backup_with_multi_region_encryption_key.go:17, tag spanner_restore_backup_with_MR_CMEK

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[telpirion]

Overall this looks good! Some comments left. Consider adding a blank line inbetween logical groups of code.