Add a `TransactionManagerCustomizer` callback interface that can be
used to customize auto-configured `PlatformTransactionManagers`.

Also update `...transaction.*` properties under a single unified
`spring.transaction...` key since the existing auto-configurations
would often share a transaction manager (the technology specific
transaction managers are `@ConditionalOnMissingBean` and may use
a manager created by a previous auto-configuration).

See gh-7561