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-platform/spring-statemachine-platform.gradle

@@ -0,0 +1,29 @@
+plugins {
+	id 'java-platform'
+}
+
+javaPlatform {
+	allowDependencies()
+}
+
+description = 'Spring Statemachine BOM'
+
+dependencies {
+	api platform("org.springframework.boot:spring-boot-dependencies:$springBootVersion")
+	constraints {
+		api "com.google.code.findbugs:jsr305:$findbugsVersion"
+		api "com.esotericsoftware:kryo-shaded:$kryoVersion"
+		api "org.springframework.shell:spring-shell:$springShellVersion"
+		api "jakarta.persistence:jakarta.persistence-api:$jakartaPersistenceVersion"
+		api "org.eclipse.uml2:uml:$eclipseUml2UmlVersion"
+		api "org.eclipse.uml2:types:$eclipseUml2TypesVersion"
+		api "org.eclipse.uml2:common:$eclipseUml2CommonVersion"
+		api "org.eclipse.emf:org.eclipse.emf.ecore.xmi:$eclipseEmfXmiVersion"
+		api "org.eclipse.emf:org.eclipse.emf.ecore:$eclipseEmfEcoreVersion"
+		api "org.eclipse.emf:org.eclipse.emf.common:$eclipseEmfCommonVersion"
+		api "org.apache.curator:curator-recipes:$curatorVersion"
+		api "org.apache.curator:curator-test:$curatorVersion"
+		api "io.projectreactor.tools:blockhound:$reactorBlockHoundVersion"
+		api "io.projectreactor.tools:blockhound-junit-platform:$reactorBlockHoundVersion"
+	}
+}