Provide support for auto-configuring multiple beans

Auto-configuration of a single DataSource works well for the vast majority of users, but when a subsequent DataSource is required things get harder (see #7652 for one example) as all of the data sources then need to be manually configured. We'd like to make things easier by providing support for auto-configuring multiple DataSources. In terms of properties, this could look something like this:

spring.datasource.primary.url=…
spring.datasource.primary.username=…
spring.datasource.primary.password=…

spring.datasource.secondary.url=…
spring.datasource.secondary.username=…
spring.datasource.secondary.password=…

Some more design work is needed, but primary could be used as a "special" name that results in the auto-configured bean being marked as @Primary. We'd also need similar functionality for components that consume a DataSource such as JPA, transaction management, Flyway and Liquibase.

Start by reviewing the proposed spring.datasource.primary and spring.datasource.secondary properties and the related example in #7652. Define how multiple DataSources should be auto-configured, including the special primary name, then determine equivalent behavior for JPA, transaction management, Flyway, and Liquibase.