Fix flyway-database-postgresql's transactional lock setting

[wilkinsona]

What does this PR do?

The metadata for flyway-database-postgresql did not allow org.flywaydb.database.postgresql.TransactionalModel to be serialized to JSON. This resulted in a failure if the setTransactionalLock method on PostgreSQLConfigurationExtension was called in a native image.

This PR adds the necessary metadata and updates the existing test to cover this code path.

Code sections where the PR accesses files, network, docker or some external service

No additional access.

Checklist before merging

• I have considered including reachability metadata directly in the library or the framework (see our contributing document)
• I am the original author of all content provided in the pull request, and I did not copy the content from any other source (see the tests section from our contributing document)
• For all tests where I am not the sole author, I have added a comment that proves I may publish them under the specified license (see the tests section from our contributing document)
• I have properly formatted metadata files (see our formatting guide)
• I have added thorough tests (see the tests section from our contributing document)
• I have filled all places where my pull request accesses files, network, docker, or any other external service (see the section above)