At the moment you can define and set per-stream metadata at the client,
which will be string-substituted and if configured in the policy, set in
related outgoing protocol specific content like h1 headers.
This patch extends the metadata concept to also check incoming protocol-
specific content like h1 headers and where it matches the binding in the
streamtype's metadata entry, make it available to the client by name, via
a new lws_ss_get_metadata() api.
Currently warmcat.com has additional headers for
server: lwsws (well-known header name)
test-custom-header: hello (custom header name)
minimal-secure-streams test is updated to try to recover these both
in direct and -client (via proxy) versions. The corresponding metadata
part of the "mintest" stream policy from warmcat.com is
{
"srv": "server:"
}, {
"test": "test-custom-header:"
},
If built direct, or at the proxy, the stream has access to the static
policy metadata definitions and can store the rx metadata in the stream
metadata allocation, with heap-allocated a value. For client side that
talks to a proxy, only the proxy knows the policy, and it returns rx
metadata inside the serialized link to the client, which stores it on
the heap attached to the stream.
In addition an optimization for mapping static policy metadata definitions
to individual stream handle metadata is changed to match by name.
In some cases devices may be too constrained to handle JSON policies but still
want to use SS apis and methodology.
This introduces an off-by-default cmake option LWS_WITH_SECURE_STREAMS_STATIC_POLICY_ONLY,
if enabled the JSON parsing part is excluded and it's assumed the user code
provides its policy as hardcoded policy structs.
