Move complement setup stuff into the Synapse repo (#12404)

Fixes matrix-org/complement#330 (or it will, once we remove the old files).

It's not quite a lift-and-shift: I've also taken the opportunity to get rid of the custom CA that we used to use to sign the TLS certs, which has been superceded by the CA exposed by Complement.

docker/complement/conf-workers/caddy.complement.json

@@ -0,0 +1,72 @@
+{
+    "apps": {
+      "http": {
+        "servers": {
+          "srv0": {
+            "listen": [
+              ":8448"
+            ],
+            "routes": [
+              {
+                "match": [
+                  {
+                    "host": [
+                      "{{ server_name }}"
+                    ]
+                  }
+                ],
+                "handle": [
+                  {
+                    "handler": "subroute",
+                    "routes": [
+                      {
+                        "handle": [
+                          {
+                            "handler": "reverse_proxy",
+                            "upstreams": [
+                              {
+                                "dial": "localhost:8008"
+                              }
+                            ]
+                          }
+                        ]
+                      }
+                    ]
+                  }
+                ],
+                "terminal": true
+              }
+            ]
+          }
+        }
+      },
+      "tls": {
+        "automation": {
+          "policies": [
+            {
+              "subjects": [
+                "{{ server_name }}"
+              ],
+              "issuers": [
+                {
+                  "module": "internal"
+                }
+              ],
+              "on_demand": true
+            }
+          ]
+        }
+      },
+      "pki": {
+        "certificate_authorities": {
+          "local": {
+            "name": "Complement CA",
+            "root": {
+              "certificate": "/complement/ca/ca.crt",
+              "private_key": "/complement/ca/ca.key"
+            }
+          }
+        }
+      }
+    }
+  }