Add a CI workflow to build xsnippet-api executables
Add a CI workflow to build xsnippet-api executables

This will:

* create a new pre-release for every PR merged to master
* create a new release for every pushed version tag

For every release we build one binary per each supported platform
(Linux, Mac OS, Windows). Note, that while Rust itself uses static
linking for everything, we link against libpq and libc dynamically.

xsnippet/xsnippet-api,xsnippet/xsnippet-api