Ability to query version (string) from C API (FFI)

[joedrago]

I'm hooking up rav1e support into libavif, and it'd be nice if there were capi calls such as:

const char * rav1e_version_short(void);
const char * rav1e_version_full(void);

Which returned lib.rs version::short, version::full string, etc, so I could expose those from my avifCodecVersions() call.

[dwbuiten]

This should be easy to do, I will try and add it in before we drop a release (very soon).