[part2] Pass Cvar through backend

Continue the generalization of cvar happening in https://github.com/o1-labs/snarky/pull/791, now cvar is fully provided by the backend.

Note: This is not mergeable as I two blocks of logic with an exception, these will get fixed in the next PR which will remove run_state.

TODO:

[x] address the TODO I commented below (see in https://github.com/o1-labs/snarky/pull/798)
[x] do run_state as well, which will simplify a number of stuff here (https://github.com/o1-labs/snarky/pull/798)
[ ] then finally do the Mina side (see in https://github.com/o1-labs/snarky/pull/798)

o1-labs / snarky