feat: support workers in run config (#3992)

# What does this PR do?


## Test Plan
Set workers: 4 in run.yaml. Start server and observe logs multiple
times.

src/llama_stack/core/datatypes.py

@@ -473,6 +473,10 @@ class ServerConfig(BaseModel):
         "- true: Enable localhost CORS for development\n"
         "- {allow_origins: [...], allow_methods: [...], ...}: Full configuration",
     )
+    workers: int = Field(
+        default=1,
+        description="Number of workers to use for the server",
+    )
 
 
 class StackRunConfig(BaseModel):