chore: refactor server.main (#3462)

# What does this PR do?
As shown in #3421, we can scale stack to handle more RPS with k8s
replicas. This PR enables multi process stack with uvicorn --workers so
that we can achieve the same scaling without being in k8s.

To achieve that we refactor main to split out the app construction
logic. This method needs to be non-async. We created a new `Stack` class
to house impls and have a `start()` method to be called in lifespan to
start background tasks instead of starting them in the old
`construct_stack`. This way we avoid having to manage an event loop
manually.


## 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.

llama_stack/core/library_client.py

@@ -40,7 +40,7 @@ from llama_stack.core.request_headers import (
 from llama_stack.core.resolver import ProviderRegistry
 from llama_stack.core.server.routes import RouteImpls, find_matching_route, initialize_route_impls
 from llama_stack.core.stack import (
-    construct_stack,
+    Stack,
     get_stack_run_config_from_distro,
     replace_env_vars,
 )
@@ -252,7 +252,10 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
 
         try:
             self.route_impls = None
-            self.impls = await construct_stack(self.config, self.custom_provider_registry)
+
+            stack = Stack(self.config, self.custom_provider_registry)
+            await stack.initialize()
+            self.impls = stack.impls
         except ModuleNotFoundError as _e:
             cprint(_e.msg, color="red", file=sys.stderr)
             cprint(
@@ -289,6 +292,7 @@ class AsyncLlamaStackAsLibraryClient(AsyncLlamaStackClient):
             )
             raise _e
 
+        assert self.impls is not None
         if Api.telemetry in self.impls:
             setup_logger(self.impls[Api.telemetry])