All role specific proxy configuration

By default only the primary role runs the proxy. To disable the proxy
for that role, you can set `proxy: false` under it.

For other roles they default to not running the proxy, but you can
enable it by setting `proxy: true` for the role, or alternatively
setting a proxy configuration.

The proxy configuration will be merged into the root proxy configuration.

lib/kamal/configuration/docs/proxy.yml

@@ -9,6 +9,12 @@
 #
 # They are application specific, so are not shared when multiple applications
 # run on the same proxy.
+#
+# The proxy is enabled by default on the primary role, but can be disabled by
+# setting `proxy: false`.
+#
+# It is disabled by default on all other roles, but can be enabled by setting
+# `proxy: true`, or providing a proxy configuration.
 proxy:
 
   # Host