Add Finish national railway

Realtime data is also available but blocked by
motis-project/motis#473.

feeds/fi.json

@@ -0,0 +1,20 @@
+{
+    "maintainers": [
+        {
+            "name": "Volker Krause",
+            "github": "vkrause"
+        }
+    ],
+    "sources": [
+        {
+            "name": "digitraffic",
+            "type": "http",
+            "url": "https://rata.digitraffic.fi/api/v1/trains/gtfs-all.zip",
+            "license": {
+                "spdx-identifier": "CC-BY-4.0",
+                "url": "https://www.digitraffic.fi/en/railway-traffic/"
+            },
+            "fix": true
+        }
+    ]
+}