Add support for customizing layers in Gradle

This commit adds configuration to the Spring Boot Gradle plugin that
allows the names and contents of layers to be customized in the build
configuration.

Fixes gh-20296

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

@@ -0,0 +1,39 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClassName = 'com.example.Application'
+	layers {
+		layers "dependencies", "commons-dependencies", "snapshot-dependencies", "static", "app"
+		libraries {
+			layerContent("snapshot-dependencies") { coordinates { include "*:*:*SNAPSHOT" } }
+			layerContent("commons-dependencies") { coordinates { include "org.apache.commons:*" } }
+			layerContent("dependencies") { coordinates { include "*:*" } }
+		}
+		classes {
+			layerContent("static") {
+				locations {
+					include "META-INF/resources/**", "resources/**"
+					include "static/**", "public/**"
+				}
+			}
+			layerContent("app") {
+				locations {
+					include "**"
+				}
+			}
+		}
+	}
+}
+
+repositories {
+	mavenCentral()
+	maven { url "https://repository.apache.org/content/repositories/snapshots" }
+}
+
+dependencies {
+	implementation("org.apache.commons:commons-lang3:3.9")
+	implementation("commons-io:commons-io:2.7-SNAPSHOT")
+}

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

@@ -0,0 +1,19 @@
+plugins {
+	id 'java'
+	id 'org.springframework.boot' version '{version}'
+}
+
+bootJar {
+	mainClassName = 'com.example.Application'
+	layers()
+}
+
+repositories {
+	mavenCentral()
+	maven { url "https://repository.apache.org/content/repositories/snapshots" }
+}
+
+dependencies {
+	implementation("org.apache.commons:commons-lang3:3.9")
+	implementation("commons-io:commons-io:2.7-SNAPSHOT")
+}

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

@@ -11,7 +11,7 @@ bootJar {
 		}
 	}
 	if (project.hasProperty('layered') && project.getProperty('layered')) {
-		layered {
+		layers {
 			includeLayerTools = project.hasProperty('excludeTools') && project.getProperty('excludeTools') ? false : true
 		}
 	}