pysa: Add basic .pyre_configuration and taint.config for pysa.

This commit adds the initial config files needed to be able to run
Pysa on Zulip code.

.pyre_configuration

@@ -0,0 +1,14 @@
+{
+  "source_directories": ["."],
+  "taint_models_path": [
+      "stubs/taint",
+      "zulip-py3-venv/lib/pyre_check/taint/"
+  ],
+  "search_path": [
+      "stubs/",
+      "zulip-py3-venv/lib/pyre_check/stubs/"
+  ],
+  "exclude": [
+      "/srv/zulip/zulip-py3-venv/.*"
+  ]
+}

stubs/taint/taint.config

@@ -0,0 +1,6 @@
+{
+  sources: [],
+  sinks: [],
+  features: [],
+  rules: []
+}