Resolves #1176

With the changes from this PR, the samples turnstile, turnstilereactive, and showcase work as described in the reference documentation.

I didn't check the other samples interactively, but their tests are still successful.

As all samples are Spring Boot applications, I've added the Spring Boot Gradle Plugin to the SamplePlugin. This lets the build produce executable jars that can be started with java -jar as described in the documentation.

To get the Spring Shell apps for the turnstile and showcase working, I've

• enabled auto-configuration with @SpringBootApplication
• removed the application context XML (no longer required with @SpringBootApplication)
• added the spring-shell-starter
• set the property spring.shell.interactive.enabled=true
• added @CommandScan to pick up the sample commands

The other Spring Shell apps like e.g. the CD player should be fixable with the same approach.

The warning on start-up of the samples about the StateMachineAnnotationPostProcessorConfiguration can be resolved by #1157 .