Refactor Config into ProofConfig and InitConfig

[poszu]

Split the Config struct into:

• ProofConfig that contains the configuration of POST proving,
• InitConfig that contains the configuration of POST initializing.
  It contains new fields for min and max num_units and labels_per_unit. These are now checked when the proof is verified (required for implement certifier web service #146).

The proving process requires only ProofConfig, while verification - both.

⚠️ this is a breaking change in the C FFI.

[codecov]

Codecov Report

Merging #145 (f4026b9) into main (3cf5917) will increase coverage by 0.06%.
Report is 1 commits behind head on main.
The diff coverage is 98.73%.

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   94.19%   94.25%   +0.06%     
==========================================
  Files          22       22              
  Lines        2944     2978      +34     
==========================================
+ Hits         2773     2807      +34     
  Misses        171      171              

Files	Coverage Δ
ffi/src/initialization.rs	90.76% <100.00%> (-0.21%)	⬇️
ffi/src/post_impl.rs	92.79% <100.00%> (+0.45%)	⬆️
service/src/service.rs	92.30% <100.00%> (+0.20%)	⬆️
src/difficulty.rs	100.00% <100.00%> (ø)
src/prove.rs	98.20% <100.00%> (-0.02%)	⬇️
src/config.rs	82.35% <0.00%> (-5.15%)	⬇️
src/verification.rs	96.86% <98.78%> (+0.14%)	⬆️