feat(messages): allow message response in params (#339)

examples/messages.py

@@ -0,0 +1,35 @@
+from anthropic import Anthropic
+
+client = Anthropic()
+
+response = client.beta.messages.create(
+    max_tokens=1024,
+    messages=[
+        {
+            "role": "user",
+            "content": "Hello!",
+        }
+    ],
+    model="claude-2.1",
+)
+print(response)
+
+response2 = client.beta.messages.create(
+    max_tokens=1024,
+    messages=[
+        {
+            "role": "user",
+            "content": "Hello!",
+        },
+        {
+            "role": response.role,
+            "content": response.content,
+        },
+        {
+            "role": "user",
+            "content": "How are you?",
+        },
+    ],
+    model="claude-2.1",
+)
+print(response2)

src/anthropic/types/beta/message_param.py

@@ -5,12 +5,13 @@
 from typing import Union, Iterable
 from typing_extensions import Literal, Required, TypedDict
 
+from .content_block import ContentBlock
 from .text_block_param import TextBlockParam
 
 __all__ = ["MessageParam"]
 
 
 class MessageParam(TypedDict, total=False):
-    content: Required[Union[str, Iterable[TextBlockParam]]]
+    content: Required[Union[str, Iterable[Union[TextBlockParam, ContentBlock]]]]
 
     role: Required[Literal["user", "assistant"]]