Add packaging and configuration files for OBS build

This allows to build packages for various distros/architectures on push and on PRs (as CI).

Here's an example of a CI run:

https://gitlab.com/bluca/openconnect/-/pipelines/478129391

The build from pushes to the master branch will look like this and create package repositories too:

https://build.opensuse.org/package/show/home:bluca:openconnect/openconnect

The repositories have a nice setup page here:

https://software.opensuse.org//download.html?project=home%3Abluca%3Aopenconnect&package=openconnect

The bare repos are under:

https://download.opensuse.org/repositories/home:/bluca:/openconnect/

Requires setting up two webhooks, will email the details.