fix: expose Spring executor for Vaadin dev mode (#692)

[denis-anisimov]

Part of #691

[vaadin-bot]

SonarQube analysis reported 14 issues

• 5 critical
• 3 major
• 6 minor

Top 10 extra issues

Note: The following issues were found on lines that were not modified in the pull request. Because these issues can't be reported as line comments, they are summarized here:

1. VaadinServletContextInitializer.java#L183: Move constants to a class or enum.
2. VaadinServletContextInitializer.java#L730: Refactor this method to reduce its Cognitive Complexity from 21 to the 15 allowed.
3. VaadinServletContextInitializer.java#L744: Refactor this code to not nest more than 3 if/for/while/switch/try statements.
4. VaadinServletContextInitializer.java#L751: Refactor this code to not nest more than 3 if/for/while/switch/try statements.
5. VaadinServletContextInitializer.java#L757: Refactor this code to not nest more than 3 if/for/while/switch/try statements.
6. VaadinServletContextInitializer.java#L196: Define and throw a dedicated exception instead of using a generic one.
7. VaadinServletContextInitializer.java#L349: Define and throw a dedicated exception instead of using a generic one.
8. VaadinServletContextInitializer.java#L401: Define and throw a dedicated exception instead of using a generic one.
9. VaadinServletContextInitializer.java#L643: Move this method into "RouteServletContextListener".
10. VaadinServletContextInitializer.java#L651: Move this method into "WebComponentServletContextListener".