RDS: Create multiple databases

### Use Case

I have a Postgres instance with multiple databases. Ideally, I'd like to create the databases directly within the CDK.

### Proposed Solution

```kotlin
new DatabaseInstanceProps.builder()
.database(listOf("database_1", "database_2")
// Some more properties
.build()
```

### Other

As far as looking at the API, I don't see a way. I can just use the `databaseName("database_table_1")` function to create one database, but not a second.

Is there any workaround / other solution to create multiple database tables within the CDK?

* [ ] :wave: I may be able to implement this feature request
* [ ] :warning: This feature might incur a breaking change

---

This is a :rocket: Feature Request

No source files or tests are named in the issue. Start by reviewing the RDS DatabaseInstanceProps API and the existing databaseName behavior, then locate the related implementation and tests; done means one CDK database instance can create multiple PostgreSQL databases while preserving the current single-database behavior.

Written by the indexing model from the issue text.