Fixes and additional support for ssl_mode for CloudSQL module

[spica29]

The CloudSQL module had some minor issues, which are covered in this PR. These are the following:

1. "postgres_client_certificates" variable was renamed to "client_certificates" to make it more generic
2. maintenance_config validation was failing when validating var.maintenance_config.maintenance_window if the var.maintenance_config is set to null
3. support for ssl_mode with validation

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Checklist

I applicable, I acknowledge that I have:

• Read the contributing guide
• Ran terraform fmt on all modified files
• Regenerated the relevant README.md files using tools/tfdoc.py
• Made sure all relevant tests pass

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[juliocc]

thanks for this!

Left a couple of comments below.

[ludoo]

Thanks, a couple small nits then it's good to go. :)

outdated

[ludoo]

Thanks for your patience!