lib/github: fixes whitespaces in `github_curl`

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib: introduce github group

* rename github_api in github_curl
* move github_curl to github group

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
lib/github: add test suite for github_curl

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>