abtract out creation and re-flattening of justifications map into helper fns

[g11tech]

https://github.com/blockblaz/zeam/blob/main/pkgs/state-transition/src/transition.zig#L271:L280

the justifications map is loaded from flattened state's justifications_roots, and justifications_validators into a justifications map and is update while processing the votes in process_attestations.

it is then flattened again to finally update it in the state (to again justifications_roots and justifications_validators)

also abtract out creation and re-flattening of justifications map into helper fns