SpringBoot Move R2DBC ConnectionFactoryBuilder and associated classes into spring-boot

The spring-boot module contains DataSourceBuilder for JDBC but the equivalent class for R2DBC, ConnectionFactoryBuilder, is in spring-boot-autoconfigure. For consistency, we should move the builder and some of its associated classes (EmbeddedDatabaseConnection, for example) into spring-boot. Having a R2DBC EmbeddedDatabaseConnection class in spring-boot will help with #24741 as it could be used to detect a ConnectionFactory for an embedded database when determining what action to take based on the initialization mode.