Automatically choose a plaintext modulus in BFV/BGV.

[sarojaduality]

In order to use MakePackedPlaintext in BFV/BGV, a user needs to choose a prime plaintext modulus t = 1 (mod m). However, this is difficult to do currently because the user can choose the security level and have OpenFHE automatically choose m. Furthermore, the user needs some way to generate a prime congruent to 1 (mod m). This issue was inspired by this Discourse question.

[yspolyakov]

@dsuponitskiy Are you sure you want to address it in v1.1.5? Choosing the modulus automatically seems tricky to me.

[dsuponitskiy]

@yspolyakov I hoped that we could have it done along with parameter validation and seemed that we would have time for this. had been planning to talk about this issue. I guess i was wrong