Spring boot Servlet integration

[slinkydeveloper]

Right now the spring boot integration uses the vert.x http server. This adds a dependency that you wouldn't use otherwise in spring. We need to implement Spring Servlet interface directly, for better integration with the spring ecosystem.

[slinkydeveloper]

After some playing around, i have a more or less prototype here: https://github.com/restatedev/sdk-java/tree/spring-boot-native
I tried few bits:

• Providing a servlet implementation. Seemed more or less ok, but haven't tested it
• Providing a reactive implementation. This one misses a crucial bit: in the webflux spec, there is no way to get from the http request the http version it was created with. This is quite a problem wrt bidirectional streaming support on the sdk side, so it's a blocker to make progress there.