Adds an example for NXP RT595S eval board, using serialized SS over CDC /
ACM USB composite device, one ttyACM for logs and the other for the SSS
link.
lws_sequencer and lws_abstract were both false starts trying to do the
functionality of secure streams.
Since Secure Streams does a better job for both and there are no known
out-of-tree users of them, let's remove them and focus on Secure Streams.
This is a NOP for existing usecases.
At the moment the only implemented transport for serialized SS is wsi, it's
typically used with Unix Domain Sockets, but it also works over tcp the
same.
It generalizes the interface between serialized chunks and the
transport, separately for client and proxy. The wsi transport is migrated
to use the new transport ops structs.
It will then be possible to "bring your own transport", so long as it is
reliable, and in-order, both for proxy and client / sspc.
We also adapt minimal-secure-streams-binance to build the -client variant
via SS proxy as well.
LWS_ONLY_SSPC is added so libwebsockets can be produced with just sspc
client support even for tiny targets.
A new embedded minimal example for rpi pico is also provided that
demonstrates using Serialized SS over a UART to an SS proxy, to implement
the SS Binance example on the pico, even though it has no networking itself.
