Consider using dlopen with gnome-secret provider

[ehuss]

@joshtriplett had the suggestion that we could change the gnome-secret provider to use dlopen to access the libsecret-1 shared library at runtime to sidestep the build issues. This would allow including it built-in to cargo without needing to worry about the build environment, or requiring it to be present on the user's system if they aren't using it. I don't know what the security implications of that might be, or how difficult it might be, but might be something to simplify the experience.