Remove usage of tmjson

[rootulp]

Context

celestiaorg/celestia-node#3935

Problem

This repo currently relies on github.com/tendermint/tendermint/libs/json (a.k.a cmtjson and tmjson) and celestia-node is interested in removing that dependency.

Proposal

• Remove usage of github.com/tendermint/tendermint/libs/json. Consider cherry-picking rpc: use encoding/json rather than tmjson tendermint/tendermint#7670
• Ensure that int64s are serialized as strings in JSON responses

cc: @renaynay @cmwaters