Aurora Discovery should support discovering the default database

[wparr-circle]

We found that the Aurora Discovery integration, does not correctly determine the default database from upstream aws api. This is set on instance creation via DBName option on the CreateDBInstance api.

This means it's not possible to correctly configure the postgres integration, where either the default postgres database is not used nor if there are multiple non-standard database names across cloud real estate.

We should support discovering this value whilst discovering cluster endpoints via https://github.com/DataDog/datadog-agent/blob/main/pkg/databasemonitoring/aws/aurora.go, which is returned by AWS API DescribeDBInstances via the same property DBName.

Setting this as a variable for templating such as extra_dbname