Configure HTTP proxy in file config (#18686)

This PR makes it possible to configure the HTTP proxy on a per-homeserver-tenant basis.

`http_proxy`, `https_proxy`, `no_proxy_hosts`

schema/synapse-config.schema.yaml

@@ -629,6 +629,33 @@ properties:
         password: mypassword
         ssh_priv_key_path: CONFDIR/id_rsa
         ssh_pub_key_path: CONFDIR/id_rsa.pub
+  http_proxy:
+    type: ["string", "null"]
+    description: >-
+      Proxy server to use for HTTP requests.
+
+      For more details, see the [forward proxy documentation](../../setup/forward_proxy.md).
+    examples:
+      - "http://USERNAME:PASSWORD@10.0.1.1:8080/"
+  https_proxy:
+    type: ["string", "null"]
+    description: >-
+      Proxy server to use for HTTPS requests.
+
+      For more details, see the [forward proxy documentation](../../setup/forward_proxy.md).
+    examples:
+      - "http://USERNAME:PASSWORD@proxy.example.com:8080/"
+  no_proxy_hosts:
+    type: array
+    description: >-
+      List of hosts, IP addresses, or IP ranges in CIDR format which should not use the
+      proxy. Synapse will directly connect to these hosts.
+
+      For more details, see the [forward proxy documentation](../../setup/forward_proxy.md).
+    examples:
+      - - master.hostname.example.com
+        - 10.1.0.0/16
+        - 172.30.0.0/16
   dummy_events_threshold:
     type: integer
     description: >-