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-test/spring-statemachine-test.gradle

@@ -0,0 +1,19 @@
+plugins {
+	id 'org.springframework.statemachine.module'
+}
+
+description = 'Spring State Machine Test'
+
+dependencies {
+	management platform(project(":spring-statemachine-platform"))
+	api project(':spring-statemachine-core')
+	api 'org.springframework:spring-test'
+	api 'org.hamcrest:hamcrest-core'
+	api 'org.hamcrest:hamcrest-library'
+	api 'org.assertj:assertj-core'
+	optional 'junit:junit'
+	optional 'org.junit.jupiter:junit-jupiter-api'
+	optional 'org.junit.vintage:junit-vintage-engine'
+	testImplementation(testFixtures(project(':spring-statemachine-core')))
+	testImplementation 'org.mockito:mockito-core'
+}