Support for server_ca_mode (in postgresql module)

### TL;DR

`sql_database_instance.settings.ip_configuration` has a field `server_ca_mode` to enable the alternative CA structures described in https://cloud.google.com/sql/docs/postgres/authorize-ssl, but this option is not exposed by the `postgresql` module.

### Terraform Resources

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### Detailed design

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### Additional information

_No response_