Previously, the embedded database connection that would be used
could only be controlled via the classpath. If multiple embedded
database dependencies were present, it wasn't possible to control
the one that the auto-configured would use. It also wasn't possible
to disable auto-configuration of an embedded database.

This commit introduces a new configuration property,
spring.datasource.embedded-database-connection. It can be set to
one of the values of the EmbeddedDatabaseConnection enum to control
the auto-configuration of an embedded database. Setting it to none
will disable the auto-configuration and ensure that an external
database is used instead.

Closes gh-23412