feat: ipv6 support in ooni-backendproxy (#61)

The `api.dev.ooni.org` domain was reportedly failing with the check-in
API as reported here: ooni/probe#2741. On more
inspection, we found that the `nginx` server was not configured with
IPv6 support which caused the connection between the upstream server and
the EC2 instance to time out. The `/var/log/nginx/error.log` logs found
on the machine:
```
2024/06/05 09:07:33 [error] 89061#89061: *519419 upstream timed out (110: Unknown error) while connecting to upstream, client: 10.0.1.164, server: _, request: "POST /api/v1/check-in HTTP/1.1", upstream: "https://[2a01:4f9:1a:9494::2]:443/api/v1/check-in", host: "api.dev.ooni.io"
2024/06/05 09:11:34 [error] 89061#89061: *519456 upstream timed out (110: Unknown error) while connecting to upstream, client: 10.0.1.164, server: _, request: "POST /api/v1/check-in HTTP/1.1", upstream: "https://[2a01:4f9:1a:9494::2]:443/api/v1/check-in", host: "api.dev.ooni.io"
```
To this end, the diff here adds support for IPv6 `egress` traffic for
the `nginx_sg` security group.

tf/modules/ooni_backendproxy/main.tf

@@ -33,6 +33,13 @@ resource "aws_security_group" "nginx_sg" {
       "0.0.0.0/0",
     ]
   }
+
+  egress {
+    from_port        = 0
+    to_port          = 0
+    protocol         = "-1"
+    ipv6_cidr_blocks = ["::/0"]
+  }
 
   lifecycle {
     create_before_destroy = true

tf/modules/ooni_backendproxy/templates/setup-backend-proxy.sh

@@ -16,6 +16,8 @@ server {
         proxy_http_version 1.1;
         proxy_set_header Host \$host;
     }
+
+    error_log /var/log/nginx/error.log;
 }
 EOF
 sudo mv $tmpfile /etc/nginx/sites-available/default