Fix missing optional argument definitions in PostKwargs

This commit introduces `params` and `files` attributes to the PostKwargs class in fasthttp.py, enhancing static type analysis capabilities without affecting existing functionality. These additions ensure more precise argument definitions for methods in FastHttpSession.

Author: kairi003

Diff for: locust/contrib/fasthttp.py

@@ -49,6 +49,8 @@ class PostKwargs(TypedDict, total=False):
         auth: tuple[str | bytes, str | bytes] | None
         allow_redirects: bool
         context: dict
+        params: dict | None
+        files: dict | None
 
     class PutKwargs(PostKwargs, total=False):
         json: Any