HTTP client API is missing

[zephyrbot]

Reported by Jukka Rissanen:

The current http_client sample application is too low level to be useful for applications.

Create a HTTP client library API that applications can use to connect to a HTTP server.

(Imported from Jira ZEP-2100)

[zephyrbot]

by Jukka Rissanen:

See #50 for details.