Overhaul gradle build

- Focus of this commit is to have modern gradle build.
- Migrate most of a plugin configurations from dsl
  into buildSrc.
- This fixes issues with existing docs build.
- Allows to sign files so that we have that part
  done for central in a build.
- We can skip publishing samples.
- We're able to share similar logic for modules
  which are meant for publish or just being samples.
- It's easier to upgrade gradle versions without
  getting various build issues.
- Relates #1143

spring-statemachine-starter/spring-statemachine-starter.gradle

@@ -0,0 +1,11 @@
+plugins {
+	id 'org.springframework.statemachine.starter'
+}
+
+description = 'Spring Statemachine Starter'
+
+dependencies {
+	management platform(project(":spring-statemachine-platform"))
+	api(project(":spring-statemachine-autoconfigure"))
+	api 'org.springframework.boot:spring-boot-starter'
+}