feat: add lifespan support to low-level MCP server

Adds a context manager based lifespan API in mcp.server.lowlevel.server to manage server lifecycles in a
type-safe way. This enables servers to:
- Initialize resources on startup and clean them up on shutdown
- Pass context data from startup to request handlers
- Support async startup/shutdown operations

src/mcp/shared/context.py

@@ -1,5 +1,5 @@
 from dataclasses import dataclass
-from typing import Generic, TypeVar
+from typing import Any, Generic, TypeVar
 
 from mcp.shared.session import BaseSession
 from mcp.types import RequestId, RequestParams
@@ -12,3 +12,4 @@ class RequestContext(Generic[SessionT]):
     request_id: RequestId
     meta: RequestParams.Meta | None
     session: SessionT
+    lifespan_context: Any