Custom username/password properties are not picked up by DataSourceInitializationConfiguration

[philwebb]

The determineDataSource doesn't return the build result so the custom username/passwords are not applied. DataSourceInitializationIntegrationTests would have caught this had it not been for the fact that #26644 was causing a ClassCastException rather than the expected "invalid authorization" error.

[philwebb]

Reopening because the org.springframework.boot.autoconfigure.sql.init.DataSourceInitializationConfiguration class has the same issue as does org.springframework.boot.autoconfigure.sql.init.R2dbcInitializationConfiguration.