feat: serialize state with protobuf

[tzdybal]

• move State to types package
• create protobuf definition for State
• add FromProto and ToProto methods
• fill validator sets in tests
• add scripts to fetch proto files from tendermint repo

Resolves #421.

Related to #37.

[codecov-commenter]

Codecov Report

Merging #424 (9113e69) into main (47f481b) will decrease coverage by 1.61%.
The diff coverage is 41.48%.

@@            Coverage Diff             @@
##             main     #424      +/-   ##
==========================================
- Coverage   56.56%   54.95%   -1.62%     
==========================================
  Files          47       48       +1     
  Lines        7858     8747     +889     
==========================================
+ Hits         4445     4807     +362     
- Misses       2777     3207     +430     
- Partials      636      733      +97     

Impacted Files	Coverage Δ
types/state.go	92.10% <ø> (ø)
types/pb/optimint/state.pb.go	38.73% <38.73%> (ø)
block/manager.go	62.38% <57.14%> (ø)
types/serialization.go	69.23% <65.00%> (-1.46%)	⬇️
state/executor.go	71.29% <66.66%> (ø)
store/store.go	65.78% <68.75%> (-2.27%)	⬇️
rpc/json/service.go	65.64% <0.00%> (+1.84%)	⬆️

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[adlerjohn]

utACK