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

[simongdavies]

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