refactor: config is now a private package (#164)

We are working on ooniprobe for Debian. Before starting to apply
changes to the codebase, I'd like to apply some refactoring steps
that I've been thinking about for quite some time.

The general concept here is that the purpose of this repository
changed since it was designed and now there is probe-engine which
is a library, therefore, this repo can be mostly private.

internal/config/testdata/config-v0.json

@@ -0,0 +1,62 @@
+{
+  "_version": 0,
+  "_informed_consent": true,
+  "_is_beta": true,
+  "auto_update": true,
+  "sharing": {
+    "include_ip": false,
+    "include_asn": true,
+    "include_country": true,
+    "include_gps": true,
+    "upload_results": true
+  },
+  "notifications": {
+    "enabled": true,
+    "notify_on_test_completion": true,
+    "notify_on_news": false
+  },
+  "automated_testing": {
+    "enabled": false,
+    "enabled_tests": [
+      "web-connectivity",
+      "facebook-messenger",
+      "whatsapp",
+      "telegram",
+      "dash",
+      "ndt",
+      "http-invalid-request-line",
+      "http-header-field-manipulation"
+    ],
+    "monthly_allowance": "300MB"
+  },
+  "test_settings": {
+    "websites": {
+      "enabled_categories": []
+    },
+    "instant_messaging": {
+      "enabled_tests": [
+        "facebook-messenger",
+        "whatsapp",
+        "telegram"
+      ]
+    },
+    "performance": {
+      "ndt_server": "auto",
+      "ndt_server_port": "auto",
+      "dash_server": "auto",
+      "dash_server_port": "auto"
+    },
+    "middlebox": {
+      "enabled_tests": [
+        "http-invalid-request-line",
+        "http-header-field-manipulation"
+      ]
+    }
+  },
+  "advanced": {
+    "use_domain_fronting": false,
+    "send_crash_reports": true,
+    "collector_url": "",
+    "bouncer_url": "https://bouncer.ooni.io"
+  }
+}

internal/config/testdata/valid-config.json

@@ -0,0 +1,19 @@
+{
+  "_version": 1,
+  "_informed_consent": false,
+  "sharing": {
+    "include_ip": false,
+    "include_asn": true,
+    "include_country": true,
+    "upload_results": true
+  },
+  "nettests": {
+    "websites_url_limit": 0
+  },
+  "advanced": {
+    "use_domain_fronting": false,
+    "send_crash_reports": true,
+    "collector_url": "",
+    "bouncer_url": "https://bouncer.ooni.io"
+  }
+}