refactor(pb/proof.go)!: converting pb definition to use snake_case

[distractedm1nd]

Related: https://github.com/celestiaorg/celestia-node/issues/2631, https://github.com/celestiaorg/celestia-node/pull/2633

[codecov[bot]]

Codecov Report

Merging #232 (f71917d) into master (9d22de9) will not change coverage. Report is 3 commits behind head on master. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #232   +/-   ##
=======================================
  Coverage   65.20%   65.20%           
=======================================
  Files           6        6           
  Lines         891      891           
=======================================
  Hits          581      581           
  Misses        293      293           
  Partials       17       17           

Files Changed	Coverage Δ
pb/proof.pb.go	0.72% <ø> (ø)