go/consensus: Expose read-only state via light client interface

[kostko]

Fixes #3077

Nodes configured as consensus RPC services workers now expose read-only
access to consensus state via the usual MKVS ReadSyncer interface, allowing
light clients to remotely query state while transparently verifying proofs.

TODO

• Expose syncer.ReadSyncer interface into consensus state over the light client interface.
• Tests.

[codecov]

Codecov Report

Merging #3078 into master will decrease coverage by 0.09%.
The diff coverage is 78.49%.

@@            Coverage Diff             @@
##           master    #3078      +/-   ##
==========================================
- Coverage   68.58%   68.48%   -0.10%     
==========================================
  Files         372      372              
  Lines       36646    36734      +88     
==========================================
+ Hits        25133    25158      +25     
- Misses       8278     8329      +51     
- Partials     3235     3247      +12     

Impacted Files	Coverage Δ
go/consensus/api/api.go	50.00% <ø> (ø)
go/consensus/api/grpc.go	65.85% <66.03%> (+0.03%)	⬆️
go/consensus/tendermint/api/api.go	75.75% <83.33%> (+1.21%)	⬆️
go/consensus/tendermint/abci/mux.go	72.81% <100.00%> (-1.16%)	⬇️
go/consensus/tendermint/full.go	61.11% <100.00%> (+2.28%)	⬆️
go/consensus/tendermint/tendermint.go	65.13% <100.00%> (+0.25%)	⬆️
go/consensus/tests/tester.go	97.80% <100.00%> (+0.65%)	⬆️
go/worker/compute/executor/committee/state.go	74.07% <0.00%> (-11.12%)	⬇️
go/consensus/tendermint/apps/roothash/query.go	68.42% <0.00%> (-10.53%)	⬇️
go/consensus/tendermint/abci/state/state.go	61.53% <0.00%> (-7.70%)	⬇️
... and 28 more

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