Multiple data sources: allow renaming default data source (feature request)

When using multiple data sources, there must be a default data source and it must be called 'dataSource'. In a project using several data sources, "dataSource" is not a useful semantic name. As a developer, I would like to be able to rename the default data source.

I would expect this to be a configuration parameter. There is already a GORM syntax for referring to the default data source, so I would expect that to continue to work. I would also expect an incorrectly configured system to produce an intelligible error message (see #10144 for an example of the current stack trace).

Example configuration syntax

```
dataSources:
default: sourceTwo
sourceOne:
driverClassName: org.h2.Driver
...
sourceTwo:
driverClassName: org.h2.Driver
...
sourceThree:
driverClassName: org.h2.Driver
...
```

Begin with the dataSources configuration handling and existing GORM syntax for the default data source; use issue #10144 to understand the current failure. Done means the default source can be renamed through configuration, existing default-source references still work, and invalid configuration produces an intelligible error.

Written by the indexing model from the issue text.