shape of any_stream?

[vinniefalco]

Currently, the native router is declared as

router< asio_route_params< AsyncStream > >

Where the type of AsyncStream can vary depending on:

• The Executor type: strand or io_context::executor_type
• Plain versus SSL (or a single stream type that supports both)
• Unix domain socket versus TCP

We need to explore offering

asio_route_params< any_stream >

Where any_stream supports all of the variations above.