Weak Link a default implementation of `c_guest_dispatch_function` in `hyperlight_guest_capi`

Implement a default implementation of `hyperlight_guest_capi` and then weak link it so a guest is not forced to provide a default implementation.

Since Rust weak linkage is an unstable feature, we should consider adding this as a C implementation with `__attribute__((weak))` in the meantime.

See [here](https://github.com/hyperlight-dev/hyperlight/blob/main/src/hyperlight_guest_capi/src/dispatch.rs#L49)