feat: allow multiple KMS keys to create CMEK database/backup

[hwin16]

Add kms_key_names field to create database/create backup code snippets.

[snippet-bot]

Here is the summary of changes.

You are about to add 4 region tags.

• samples/samples/backup_sample.py:119, tag spanner_create_backup_with_MR_CMEK
• samples/samples/backup_sample.py:249, tag spanner_restore_backup_with_MR_CMEK
• samples/samples/backup_sample.py:700, tag spanner_copy_backup_with_MR_CMEK
• samples/samples/snippets.py:337, tag spanner_create_database_with_MR_CMEK

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[hwin16]

I am a googler following self-serve guide.

[harshachinta]

@hwin16 The handwritten changes are not needed since it is a admin feature.
The samples look good. Will approve the PR once the handwritten changes are removed.

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
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[hwin16]

Thank you @harshachinta! PTAL.

[harshachinta]

@hwin16 Can you please fix the lint by running nox -s blacken in samples/samples folder.
Also sample tests are failing. please fix

[hwin16]

@harshachinta , I have fixed the linting issue. I fixed some issues with integration tests but wasn't able to run them. Our feature is adding repeated field to proto and similar to existing unit test "create_database_with_encryption_key". When I run existing unit test, that is failing without any changes. Looks like instance set up is incorrect in integration test?

[harshachinta]

@hwin16
This test is dependent on another test that creates the instance once. The dependency annotation is missing for these tests. Let me fix it in seperate PR.
However you can run the following command to get it working,

nox -s py-3.11 -- -k "test_create_database_explicit or test_create_database_with_encryption_config"

[hwin16]

@harshachinta , done fixing integration tests and they are passing now. Currently, skipped them before feature becomes public.

Passing test results:

[harshachinta]

Do we have a fixture for this multi_region_instance?
Where is this instance getting create? I am not able to find in this code

[harshachinta]

LGTM with couple of questions. Once the backend is ready we can merge this PR.