Attach updated endpoint signature to inner

Not all endpoints accept a __signature__ attribute, nor should the
cache decorator modify the decorated endpoint. Attach the signature
to the returned inner function instead.

While here, refactor the signature updating code, and extract it to
a separate function.

fastapi_cache/decorator.py

@@ -22,6 +22,36 @@ P = ParamSpec("P")
 R = TypeVar("R")
 
 
+def _augment_signature(
+    signature: inspect.Signature, add_request: bool, add_response: bool
+) -> inspect.Signature:
+    if not (add_request or add_response):
+        return signature
+
+    parameters = list(signature.parameters.values())
+    variadic_keyword_params = []
+    while parameters and parameters[-1].kind is inspect.Parameter.VAR_KEYWORD:
+        variadic_keyword_params.append(parameters.pop())
+
+    if add_request:
+        parameters.append(
+            inspect.Parameter(
+                name="request",
+                annotation=Request,
+                kind=inspect.Parameter.KEYWORD_ONLY,
+            ),
+        )
+    if add_response:
+        parameters.append(
+            inspect.Parameter(
+                name="response",
+                annotation=Response,
+                kind=inspect.Parameter.KEYWORD_ONLY,
+            ),
+        )
+    return signature.replace(parameters=[*parameters, *variadic_keyword_params])
+
+
 def cache(
     expire: Optional[int] = None,
     coder: Optional[Type[Coder]] = None,
@@ -48,33 +78,6 @@ def cache(
             (param for param in signature.parameters.values() if param.annotation is Response),
             None,
         )
-        parameters = []
-        extra_params = []
-        for p in signature.parameters.values():
-            if p.kind <= inspect.Parameter.KEYWORD_ONLY:
-                parameters.append(p)
-            else:
-                extra_params.append(p)
-        if not request_param:
-            parameters.append(
-                inspect.Parameter(
-                    name="request",
-                    annotation=Request,
-                    kind=inspect.Parameter.KEYWORD_ONLY,
-                ),
-            )
-        if not response_param:
-            parameters.append(
-                inspect.Parameter(
-                    name="response",
-                    annotation=Response,
-                    kind=inspect.Parameter.KEYWORD_ONLY,
-                ),
-            )
-        parameters.extend(extra_params)
-        if parameters:
-            signature = signature.replace(parameters=parameters)
-        func.__signature__ = signature
 
         @wraps(func)
         async def inner(*args: P.args, **kwargs: P.kwargs) -> R:
@@ -179,6 +182,9 @@ def cache(
             response.headers["ETag"] = etag
             return ret
 
+        inner.__signature__ = _augment_signature(
+            signature, request_param is None, response_param is None
+        )
         return inner
 
     return wrapper