Adding DiffSuppress for google_sql_database_instance on disk_type

[NA2047]

Potential fix for #10492

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[melinath]

I'm not sure if this will be reusable; for now it might be preferable to have a very specific diff suppress func for this field that uses strings.Replace to convert dashes to underscore - unless the API supports any delimiter?

If a specific diff suppress func would work, it should be defined in the resource file instead of the common file.

Either way - please add some unit tests for the diff suppress func. See common_diff_suppress_test.go for examples. Thanks!

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