Update spec to add support for alt_bn128_g1_multiexp, alt_bn128_g1_sum, and alt_bn128_pairing_check functions

[akhi3030]

#98 is the issue where the proposal was discussed. We did not follow the NEP process for this work as the bulk of the work was done before the NEP process was stabilised. In near/nearcore#6824, we added support for the functions in nearcore. This issue is to track the remaining work namely updating the spec.