add the ability to wrap stdlib sockets

[graingert]

Things to check first

• I have searched the existing issues and didn't find my feature already requested there

Feature description

I'd like the ability to wrap a stdlib socket in a SocketStream/UNIXSocketStream/SocketListener/UDPSocket/ConnectedUDPSocket

Ideally it would be a synchronous function that takes a connected or listening socket of any kind and does the right thing, but asyncio only provides async methods to adopt a socket. It's also hard to detect if a socket is connected or connecting or listening securely for the purposes of wrapping with TLS

Perhaps the best interface for now would be two async functions, async def wrap_client_sock -> SocketStream | UNIXSocketStream | ConnectedUDPSocket and async def wrap_server_sock -> SocketListener | UDPSocket ?

Use case

I plan to use it with socket.socketpair in the anyio.to_process.run_sync implementation to fix #516