HashType and CommitmentType specification

[codygunton]

We have lines like

static constexpr merkle::HashType merkle_hash_type = merkle::HashType::LOOKUP_PEDERSEN;
static constexpr pedersen::CommitmentType commitment_type = pedersen::CommitmentType::FIXED_BASE_PEDERSEN;

in the circuit builders, and there is a question of where this sort of data actually belongs. Should it go in the flavor? My hunch is that it's probably better not to attach this choice once and for all to a proving system, but to select the type on a per-application basis. Someone should dig in and figure out the proper place of this (maybe it is actually Aztec-specific and shouldn't be in Barretenberg at all?). Perhaps this can go away altogether with the introduction of the one true Pedersen hash in #671?