v2alpha1: Add PostService

[kacpersaw]

PostConfig is missing in v2alpha1. This PR introduces new service which returns post config.

message PostConfigResponse {
  uint32 bits_per_label = 1;
  uint64 labels_per_unit = 2;
  uint32 min_num_units = 3;
  uint32 max_num_units = 4;
  uint32 k1 = 5;
  uint32 k2 = 6;
}

Draft until https://github.com/spacemeshos/api/pull/364 is merged