[otbn,dv] Parameterize `otbn_intg_err` base class

[ctopal]

When otbn_intg_err got implemented it was explicitly used for injecting integrity errors to bignum registers. Therefore the arguments for the some tasks and functions are written for BigNum lengths. If we can parameterize it, we then don't need to have otbn_rf_base_intg_err implement it's own body.

[rswarbrick]

I'm convinced this is a tidyup worth doing. And I think we might even be able to do it without all the parameterisation described above. But I'm removing the M4 milestone: while it's worth doing, I don't believe this is something we should delay a tapeout milestone for.