* Mark google_sql_database.{charset,collation} as computed instead of having defaults.
This change is required to avoid the following scenario:
When upgrading from a previous version of the Google provider, TF will change
the charset/collation of existing (TF-managed) databases to utf8/utf8_general_ci
(if the user hasn't added different config values before running TF apply),
potentially overriding any non-default settings that the user my have applied
through the Cloud SQL admin API. This violates POLA.
* Remove charset/collation defaults from the documentation, too.
* Add links to MySQL's and PostgreSQL's documentation about supported charset and collation values.
* Use version 5.7's docs instead of 5.6, since that's the most up to date version of MySQL that we support.
* Add a note that only UTF8 / en_US.UTF8 are currently supported for Cloud SQL PostgreSQL databases.
* Add charset and collation to google_sql_database.
* Add documentation for charset, collation attributes.
* Extend the existing acceptance test to also cover charset and collation.
* Charset and collation always have a value present. Also inline.
* Move charset and collation to optional arguments.
* Add charset and collection to the example.
* Document charset and collation defaults.
* Keep TestAccGoogleSqlDatabase_basic as is, add TestAccGoogleSqlDatabase_update.
