Pipelined Guardrails (#32)

* initial draft: pipelined guardrails

* documentation on stream instrumentation

* more comments

* fix: return earlier

* non-streaming case

* handle non-streaming case

* fix more cases

* simplify request instrumentation

* improve comments

* fix import issues

* extend tests for input guardrailing

* anthropic integration of pipelined and pre-guardrailing

* fix gemini streamed refusal

example_policy.gr

@@ -0,0 +1,7 @@
+from invariant.detectors import prompt_injection
+
+raise "Don't say 'Hello'" if:
+    (msg: Message)
+    msg.role == "user"
+    prompt_injection(msg.content)
+    # "Hello" in msg.content