Add mainClassName to springBoot DSL in Gradle plugin

This commit introduces a new mainClassName property on the springBoot
DSL provided by the Gradle plugin. It can be used to explicitly
configure the mainClassName of the default bootRun, bootJar, and
bootWar tasks in a single place. Previously, the only mechanism
provided to do so was the mainClassName property that's only available
when the application plugin has been applied.

Closes gh-10623

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootJarIntegrationTests-springBootExtensionMainClassNameIsUsed.gradle

@@ -0,0 +1,13 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'java'
+apply plugin: 'org.springframework.boot'
+apply plugin: 'application'
+
+springBoot {
+	mainClassName = 'com.example.CustomMain'
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/bundling/BootWarIntegrationTests-springBootExtensionMainClassNameIsUsed.gradle

@@ -0,0 +1,13 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'war'
+apply plugin: 'org.springframework.boot'
+apply plugin: 'application'
+
+springBoot {
+	mainClassName = 'com.example.CustomMain'
+}

spring-boot-project/spring-boot-tools/spring-boot-gradle-plugin/src/test/resources/org/springframework/boot/gradle/tasks/run/BootRunIntegrationTests-springBootExtensionMainClassNameIsUsed.gradle

@@ -0,0 +1,16 @@
+buildscript {
+	dependencies {
+		classpath files(pluginClasspath.split(','))
+	}
+}
+
+apply plugin: 'application'
+apply plugin: 'org.springframework.boot'
+
+springBoot {
+	mainClassName = 'com.example.CustomMainClass'
+}
+
+task echoMainClassName {
+	println 'Main class name = ' + bootRun.mainClassName
+}