Creates random passwords for additional_users

[samcre]

For every Cloud SQL instance type, if additional_users.password it's
an empty string, the module will create a random password.

Also, adds an output to retrieve users and passwords from other
resources.

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[comment-bot-dev]

Thanks for the PR! 🚀
✅ Lint checks have passed.

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[samcre]

@googlebot I signed it!

[morgante]

@samcre Please address the formatting issues.

[samcre]

@morgante I think I already did it, but terraform-google-sql-db--int-trigger is failing again, and I can't see what's going on with the build. 🤔

[morgante]

Thanks for working on this, just one nit.

[morgante]

@samcre Looks like the test failure is unrelated, you can ignore it.

[samcre]

@morgante Everything is OK? Should I make more changes?

[tfluehmann]

HI @samcre
Will this change overwrite existing passwords?