Precompute discrete log lookup

**Description**
This is the sibling issue to [the python issue](https://github.com/microsoft/electionguard-python/issues/227)

currently, `elgamal.cpp#decrypt` only decrypts a zero or a one. this issue is to implement the dlog in the same way as python, and to set a cutoff.

Additionally, we would like to precompute the values up to the cutoff and supply them as an optional file along with the binary so that consumers can choose to optimize for compute performance at the expense of storage space.

the relevant python code was updated as part of [PR 405](https://github.com/microsoft/electionguard-python/pull/405) and the same pattern should be applied here.