Support Query Insights for MySQL

[thealgirdas]

Issue https://github.com/terraform-google-modules/terraform-google-sql-db/issues/347

GCP now support Query Insights for MySQL

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[thealgirdas]

@bharathkkb please review when ready! This resolves https://github.com/terraform-google-modules/terraform-google-sql-db/issues/347

[alexrjones]

@thealgirdas Would you be able to add support for the safer_mysql module? Should just require exposing the insights_config variable in safer_mysql/variables.tf and passing it to the mysql module in safer_mysql/main.tf. Otherwise I will do this in a separate PR.

[comment-bot-dev]

@thealgirdas Thanks for the PR! 🚀
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