Make helper functions for adding accounts to genesis state.

[ValarDragon]

Summary

Right now, its quite intensive to add a new Account with a given balance to the genesis state. (e.g. the map[string]json.RawMessage representation)

In Osmosis. this (logically) simple operation is a 40 line endeavour: https://github.com/osmosis-labs/osmosis/blob/main/tests/e2e/chain/config.go#L95-L134

Proposal

Make a helper function(s) in either Auth, Bank, or both, to significantly ease the complexity of adding a new account to an existing appGenesisState. There are numerous occassions that I think this will come in handy, outside of Osmosis' e2e testing. (e.g. airdrop / genesis file building frameworks)

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