Add remote shell implementation based on crsh

This commit adds a new starter named spring-boot-starter-shell-crsh and auto configuration support to embed a system shell within Spring Boot applications.

The embedded shell allows clients to connect via ssh or telnet to the Boot app and execute commands. Commands can be implemented and embedded with app.

For sample usage see spring-boot-samples-actuator.

spring-boot-samples/spring-boot-sample-actuator/src/main/resources/application.properties

@@ -6,4 +6,12 @@ server.port: 8080
 server.tomcat.basedir: target/tomcat
 server.tomcat.access_log_pattern: %h %t "%r" %s %b
 security.require_ssl: false
-service.name: Phil
+service.name: Phil
+shell.ssh.enabled: true
+shell.ssh.port: 2222
+shell.telnet.enabled: false
+#shell.telnet.port: 1111
+shell.auth: spring
+#shell.auth: key
+#shell.auth.key.path: ${user.home}/test/id_rsa.pub.pem
+#shell.auth: simple