Add support for using RemoteSpringApplication behind a proxy

This commit adds two new properties, spring.devtools.remote.proxy.host
and spring.devtools.remote.proxy.port that can be used to configure
RemoteSpringApplication to connect to the remote application through
an HTTP proxy.

Closes gh-3968

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -674,6 +674,8 @@ content into your application; rather pick only the properties that you need.
     spring.devtools.remote.context-path=/.~~spring-boot!~ # context path used to handle the remote connection
     spring.devtools.remote.debug.enabled=true # enable remote debug support
     spring.devtools.remote.debug.local-port=8000 # local remote debug server port
+    spring.devtools.remote.proxy.host= # the host of the proxy to use to connect to the remote application
+    spring.devtools.remote.proxy.port= # the port of the proxy to use to connect to the remote application
     spring.devtools.remote.restart.enabled=true # enable remote restart
     spring.devtools.remote.secret= # a shared secret required to establish a connection
     spring.devtools.remote.secret-header-name=X-AUTH-TOKEN # HTTP header used to transfer the shared secret

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -1064,6 +1064,9 @@ property is read and passed to the server for authentication.
 TIP: It's always advisable to use `https://` as the connection protocol so that traffic is
 encrypted and passwords cannot be intercepted.
 
+TIP: If you need to use a proxy to access the remote application, configure the
+`spring.devtools.remote.proxy.host` and `spring.devtools.remote.proxy.port` properties.
+
 
 
 [[using-boot-devtools-remote-update]]