Allow Flyway or Liquibase to be auto-configured without a DataSource bean

[L00kian]

LiquibaseAutoConfiguration and FlywayAutoConfiguration both require DataSource bean to be declared. However both configurations allow to explicitly define datasource properties which would create one anyway. #16849

[philwebb]

We might refine the condition to check for Datasource or the URL property when we merge.

[wilkinsona]

Thank you very much for making your first contribution to Spring Boot, @L00kian. The changes have now been merged into master with some modifications. As suggested above by @philwebb, we now look for either a DataSource or a spring.(flyway|liquibase).url property. The auto-configuration backs off is neither is available. This reduces the scope of the changes as the custom exceptions and failure analyzers are no longer required. You can see combined effect of your changes and the modifications in the diff of the merge commit.

[L00kian]

@wilkinsona Thank you for accepting it. I was glad to contribute