Rework DataSource initialization

Previously, DataSource initialization was triggered via a
BeanPostProcessor or a schema created event from JPA. This caused
numerous problems with circular dependencies, bean lifecycle, etc and
added significant complexity.

This commit reworks DataSource initialization to remove the use of a
BeanPostProcessor entirely. In its place, DataSource initialization is
now driven by an InitializingBean with dependency relationships
between beans ensuring that initialization has been performed before
the DataSource is used. This aligns with the approach that's worked
well with Flyway and Liquibase.

More changes are planned to further simplify DataSource initialization.
The changes in this commit are a foundation for those changes. Any new
public API in this commit is highly likely to change before the next
GA.

Fixes gh-13042
Fixes gh-23736

spring-boot-project/spring-boot-docs/src/docs/asciidoc/howto.adoc

@@ -2029,6 +2029,11 @@ In addition, Spring Boot processes the `schema-$\{platform}.sql` and `data-$\{pl
 This allows you to switch to database-specific scripts if necessary.
 For example, you might choose to set it to the vendor name of the database (`hsqldb`, `h2`, `oracle`, `mysql`, `postgresql`, and so on).
 
+By default, script-based `DataSource` initialization is performed before any JPA `EntityManagerFactory` beans are created.
+`schema.sql` can be used to create the schema for JPA-managed entities and `data.sql` can be used to populate it.
+If you want script-based `DataSource` initialization to be performed after the `EntityManagerFactory`, set `spring.datasource.initialization-order` to `after-jpa`.
+`schema.sql` can then be used to make additions to any schema creation performed by Hibernate and `data.sql` can be used to populate it.
+
 [NOTE]
 ====
 When only basic SQL scripts are used, Spring Boot automatically creates the schema of an embedded `DataSource`.