Add a http client implementation.

[peterharperuk]

Implemented using the lwip http client.

Fixes #1386

[kilograham]

comment should probably explain something about what _secure means

[peterharperuk]

Other comments,

1/ Passing the hostname in http_client_init means you can't reuse the same "state" to make a request to a different host. Seems a bit restrictive? The only reason I can think for this is that your tls config might be related to the host you're talking to. It's easy enough to allocate "state" for each host?

2/ Can't run multiple requests at the same time? You could do this by calling http_client_init multiple times for the number of requests you want to have running at the same time. The lwip api just allocates a new state for each request and frees this in the callback. We could do the same but it seems wasteful when you want to make multiple requests one after another (to the same host).

[kilograham]

Other comments,

1/ Passing the hostname in http_client_init means you can't reuse the same "state" to make a request to a different host. Seems a bit restrictive? The only reason I can think for this is that your tls config might be related to the host you're talking to. It's easy enough to allocate "state" for each host?

2/ Can't run multiple requests at the same time? You could do this by calling http_client_init multiple times for the number of requests you want to have running at the same time. The lwip api just allocates a new state for each request and frees this in the callback. We could do the same but it seems wasteful when you want to make multiple requests one after another (to the same host).

Yeah, perhaps have a friendly API which combines the underlying "state" init, and the actual request, but expose the underlying methods too.

I guess I need to understand what the thing we are initializing is (it's life time) to know what it should be called (is it actually a client instance, a connection, etc.?)

[peterharperuk]

I guess I need to understand what the thing we are initializing is (it's life time)

It looks like the lwip api is per http request - which is a bit naff. This was just supposed to be a "helper" utility to make the lwip functionality a bit easier to use (and hide the lwip dependency). Originally it was just part of the example code but I thought that was a bit lazy and should maybe add it to the sdk. Maybe I should forget that.

[peterharperuk]

I pushed a new version. To make an http request is something like this...

PICO_HTTP_REQUEST_T req = {0};
req.hostname = HOST;
req.url = URL;
req.recv_fn = response_function;
int rc = http_client_request_sync(cyw43_arch_async_context(), &req);

I'm not sure this is adding a lot of value, so I might just move it back to pico-examples. What do you think?

[peterharperuk]

This has been moved into a http client example to demonstrate how to use the lwip http client