Spanner Instance Module minor fixes and improvements

[frandarre14]

Working on a project with @mauriciowillig we've noticed that the spanner_instance module was not passing the variable to the Spanner Instance Database resource within, so it was defaulting on creating the database with GOOGLE_CLOUD_SQL dialect and ignoring POSTGRES even if added as a module variable.

We've also noticed that there was no support to input instance settings like database edition, type and default backup schedule type, so we've added those as well.

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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

[frandarre14]

Will add defaults on next commit to prevent failure.