chore: refactor server.main

# What does this PR do?
Refactor main to split out the app construction so that we can use `uvicorn --workers` to enable multi-process stack.


## Test Plan
CI

> uv run --with llama-stack python -m llama_stack.core.server.server benchmarking/k8s-benchmark/stack_run_config.yaml

works.

> LLAMA_STACK_CONFIG=benchmarking/k8s-benchmark/stack_run_config.yaml uv run uvicorn llama_stack.core.server.server:create_app --port 8321 --workers 4

works.

benchmarking/k8s-benchmark/stack-configmap.yaml

@@ -5,6 +5,7 @@ data:
     image_name: kubernetes-benchmark-demo
     apis:
     - agents
+    - files
     - inference
     - files
     - safety
@@ -23,6 +24,14 @@ data:
       - provider_id: sentence-transformers
         provider_type: inline::sentence-transformers
         config: {}
+      files:
+      - provider_id: meta-reference-files
+        provider_type: inline::localfs
+        config:
+          storage_dir: ${env.FILES_STORAGE_DIR:=~/.llama/distributions/starter/files}
+          metadata_store:
+            type: sqlite
+            db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/files_metadata.db
       vector_io:
       - provider_id: ${env.ENABLE_CHROMADB:+chromadb}
         provider_type: remote::chromadb