Introduce TransactionExecutionListener with begin/commit/rollback notifications

Includes ConfigurableTransactionManager interface for listener registration.
Includes additional introspection methods on TransactionExecution interface.
Includes default method declarations on TransactionStatus/SmartTransactionObject.

Closes gh-27479

spring-jdbc/src/main/java/org/springframework/jdbc/datasource/JdbcTransactionObjectSupport.java

@@ -131,11 +131,6 @@ public abstract class JdbcTransactionObjectSupport implements SavepointManager,
 		return this.savepointAllowed;
 	}
 
-	@Override
-	public void flush() {
-		// no-op
-	}
-
 
 	//---------------------------------------------------------------------
 	// Implementation of SavepointManager