[Feature]: Bulk add genesis accounts

[Reecepbcups]

Summary

Protocols need an easy way to add / airdrop bulk account allocations into the genesis file. appd genesis add-genesis-account ... is far to slow (~100 accounts per 8-10 seconds). By bringing in a bulk command to read a JSON file, it drastically reduces the time to build a genesis. (In my case from 15 - 20 hours estimate for 250k accounts, down to ~4 seconds).

Problem Definition

• Creating large genesis files is impracticable and difficult if you have more than 5k accounts
• As the interchain grows, making better UX to distribute network tokens is important to allow developers without having to write complex scripts.
• The SDK improves UX for new chains coming up who want to distribute tokens to a subset of large network participants (i.e. CosmosHub, Osmosis)
• There are no disadvantages other than re-working the AddGenesisAccounts method in genutilstypes

Proposed Feature

A bulk-add-genesis-accounts command should be added that allows reading from a file format. An account balance cache should be added on read to reduce the excessive calls to saving genesis state when it can be done in memory.

An example I've built and used (with limited scope to fit our specific needs) is found here: https://github.com/CryptoDungeon/dungeonchain/blob/main/cmd/dungeond/fast_add_genesis_airdrop.go

The GenesisAccount struct should not be the format, but this gives the idea for what a format would look like. An address, sdk.Coins array, and other info such as vesting would be put here. Then a developer can just take a JSON (or csv?) file and load it in quickly.

[Reecepbcups]

If this is something desired please assign me & I will take it on :)
If not, I understand as well.

[tac0turtle]

This sounds good. It makes sense to add it. Just a heads up we deprecated existing vesting accounts so they can not be created any longer. New vesting accounts need to be done through a vesting account via x/accounts. Would love to review a pr if you have time thank you