fix coinbase rpc endpoint to return ethereum address of the validator

[thomas-nguy]

Closes: #112

Description

Coinbase previously was returning the operator address of the validator in hex format.

After this PR, eth_coinbase will return the ethereum format address of the account of the validator

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[fedekunze]

Logic looks good, although can be simplified (see comments). Also if you could add a test for the query on grpc_test.go 🙏

[codecov]

Codecov Report

Merging #153 (6018f68) into main (04bacbd) will decrease coverage by 0.85%.
The diff coverage is 18.82%.

❗ Current head 6018f68 differs from pull request most recent head 3b095d0. Consider uploading reports for the commit 3b095d0 to get more accurate results

@@            Coverage Diff             @@
##             main     #153      +/-   ##
==========================================
- Coverage   48.84%   47.98%   -0.86%     
==========================================
  Files          43       43              
  Lines        2848     2932      +84     
==========================================
+ Hits         1391     1407      +16     
- Misses       1389     1454      +65     
- Partials       68       71       +3     

Impacted Files	Coverage Δ
x/evm/types/query.pb.gw.go	0.00% <0.00%> (ø)
x/evm/keeper/grpc_query.go	77.51% <64.00%> (-2.35%)	⬇️
x/evm/keeper/abci.go	0.00% <0.00%> (ø)
x/evm/keeper/keeper.go	50.49% <0.00%> (ø)
x/evm/keeper/state_transition.go	0.00% <0.00%> (ø)

[fedekunze]

LGTM. Pending test and lint fixes