Add support for CloudSQL instance point-in-time recovery for selected DBs

[dapengzhang0]

This change is regarding following feature ask #13903

Introducing terraform support of single-database-recovery for SQL Server PITR with --database_names option.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• [ X] Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

Release Note Template for Downstream PRs (will be copied)

sql: added support of single-database-recovery for SQL Server PITR with `database_names` attribute to `google_sql_instance`

[slevenick]

/gcbrun

[modular-magician]

Hi there, I'm the Modular magician. I've detected the following information about your changes:

Diff report

Your PR generated some diffs in downstreams - here they are.

Terraform GA: Diff ( 3 files changed, 73 insertions(+))
Terraform Beta: Diff ( 3 files changed, 73 insertions(+))
TF Validator: Diff ( 2 files changed, 3 insertions(+), 3 deletions(-))

[modular-magician]

Tests analytics

Total tests: 2524
Passed tests 2255
Skipped tests: 265
Affected tests: 4

Action taken

Found 4 affected test(s) by replaying old test recordings. Starting RECORDING based on the most recent commit. Click here to see the affected tests

TestAccComposerEnvironment_withEncryptionConfigComposer1|TestAccNetworkServicesGateway_update|TestAccDataSourceGoogleCloudAssetResourcesSearchAll_basic|TestAccDataSourceDnsManagedZone_basic

Get to know how VCR tests work

[modular-magician]

Tests passed during RECORDING mode:
TestAccComposerEnvironment_withEncryptionConfigComposer1[Debug log]
TestAccNetworkServicesGateway_update[Debug log]

Tests failed during RECORDING mode:
TestAccDataSourceGoogleCloudAssetResourcesSearchAll_basic[Error message] [Debug log]
TestAccDataSourceDnsManagedZone_basic[Error message] [Debug log]

Please fix these to complete your PR
View the build log or the debug log for each test

[slevenick]

Question on readability of the field

[slevenick]

Do we need to read this field from the API? If so, it should be present in the flatten function for this field also.

[dapengzhang0]

I don't think so. This field should follow the same pattern as point_in_time field, which is not read from the API.