MONGOCRYPT-705 changes `mc_#Token_new_from_buffer()` and `mc_#Token_new_from_buffer_copy` to take in a const argument as the implementations only use the argument as const.

For correctness, we should change calls to these functions in the server code to pass through const buffers as well. This is blocked by updating libmongocrypt to the next minor version (after function argument changes are made). The attached patch should be all that's needed in the server.