Make base_mcmc into crtp?

[SteveBronder]

Summary:

I'm looking at base_mcmc and it looks like it's virtualness is only used in a few functions

run_sampler
generate_transitions
write_adapt_finish

I think we could rewrite that to just be a CRTP class and change those signatures to be like

template <typename Model, typename RNG, typename DerivedMCMC>
void run_sampler(stan::mcmc::base_mcmc<DerivedMCMC>& sampler, Model& model,

Expected Output:

Removal of some dynamic lookup for compile time lookup

Current Version:

v2.21.0