Add a http client

[peterharperuk]

I'm adding some test code for the lwip http client to pico-examples raspberrypi/pico-examples#318

But I think it's a bit of a cop out to leave it over there. We should really have a pico http client in the sdk that users can use until they find something better?

Some attempt to make it independent from lwip is a bonus.

[kilograham]

moving this to 1.6.0 unless there is a reason we need it for 1.5.1

[peterharperuk]

yes, probably wise