Draft: Add role for Caddy web server

Our current nginx+acmebot setup is broken in Debian stable. I am not happy with the webserver role in various points anyway, and the ACME problem still stands. Caddy solves both.

Draft, because I still need to write docs.

caddy/defaults/main.yml

+---
+
+# caddy_global:
+#   auto_https: false
+#   email: acme@example.org
+#   on_demand_tls:
+#     ask: http://localhost:9123/ask
+#   debug: ""  # Special value-less option, use with empty string
+#   http_port: null  # Omit, useful to override in more specific variables
+
+caddy_local_sites: {}
+#   example.org:
+#     root *: /var/www
+#     file_server: ""
+
+caddy_enabled_sites: []
+#   - example.com  # Config filled in by some other role
+
+caddy_local_config: {}
+# cors:
+#   "(cors)":
+#     # Note the differences in quoting!  The next line will not contain quotes
+#     # in the resulting Caddyfile, the one after that _will_ end up with
+#     # quotes!
+#     "@origin header Origin": "{args[0]}"
+#     header @origin Access-Control-Allow-Origin: '"{args[0]}"'
+#     header @origin Access-Control-Allow-Methods: >-
+#       "OPTIONS,HEAD,GET,POST,PUT,PATCH,DELETE"
+# # Or just specify the contents verbatim:
+# https-proxy: |
+#   (https-proxy) {
+#   	reverse_proxy {args[:]} {
+#   		transport http {
+#   			tls
+#   		}
+#   	}
+#   }
+
+caddy_enabled_config: []