Improve Coroutines transaction API

[sdeleuze]

As a follow-up of #22915, the purpose of this issue is to improve Coroutines programmatic transaction API to make it more consistent with the Java one.

For suspending functions, we currently have TransactionalOperator.transactional extension with a suspending lambda parameter which is conceptually closer to TransactionalOperator.execute Java API, so it would probably make sense to rename it TransactionalOperator.executeAndAwait and add a ReactiveTransaction parameter to the lambda.

For Flow the TransactionalOperator.transactional is correct but would be more idiomatic as a Flow extension.