refactor: move async dependencies from optional to direct dependencies

Based on dependency analysis, adding starlette, uvicorn, and uvicorn-worker
as direct dependencies only increases installation size by ~816KB (3.5%).
This minimal impact makes it reasonable to include them directly, simplifying
installation for users who need async functionality while maintaining
Python 3.8+ compatibility constraints.

Author: taeold

pyproject.toml

@@ -30,18 +30,14 @@ dependencies = [
   "cloudevents>=1.2.0,<=1.11.0",                  # Must support python 3.7
   "Werkzeug>=0.14,<4.0.0",
   "httpx>=0.24.1",
+  "starlette>=0.37.0,<1.0.0; python_version>='3.8'",
+  "uvicorn>=0.18.0,<1.0.0; python_version>='3.8'",
+  "uvicorn-worker>=0.2.0,<1.0.0; python_version>='3.8'",
 ]
 
 [project.urls]
 Homepage = "https://github.com/googlecloudplatform/functions-framework-python"
 
-[project.optional-dependencies]
-async = [
-    "starlette>=0.37.0,<1.0.0; python_version>='3.8'",
-    "uvicorn>=0.18.0,<1.0.0; python_version>='3.8'",
-    "uvicorn-worker>=0.2.0,<1.0.0; python_version>='3.8'"
-]
-
 [project.scripts]
 ff = "functions_framework._cli:_cli"
 functions-framework = "functions_framework._cli:_cli"