Add support for authorization_code grant

Fixes gh-4928

samples/boot/oauth2/authcodegrant/spring-security-samples-boot-oauth2-authcodegrant.gradle

@@ -0,0 +1,16 @@
+apply plugin: 'io.spring.convention.spring-sample-boot'
+
+ext['thymeleaf.version'] = '3.0.9.RELEASE'
+
+dependencies {
+	compile project(':spring-security-config')
+	compile project(':spring-security-oauth2-client')
+	compile 'org.springframework:spring-webflux'
+	compile 'org.springframework.boot:spring-boot-starter-thymeleaf'
+	compile 'org.springframework.boot:spring-boot-starter-web'
+	compile 'org.thymeleaf.extras:thymeleaf-extras-springsecurity4'
+	compile 'io.projectreactor.ipc:reactor-netty'
+
+	testCompile project(':spring-security-test')
+	testCompile 'org.springframework.boot:spring-boot-starter-test'
+}