CLI: Add support for multi-messages for each tx

[anilcse]

Summary

As a delegator if I want to delegate to multiple validators in a single transaction, there's no default way possible via CLI. Add support for CLI to generate or send multiple messages in a single transaction.

Problem Definition

Currently, though it's all possible from the protocol, CLI doesn't provide a way to generate multiple messges in single transaction. Users either have to use custom scripts or manually edit generated tx files to add multiple messages. Especially teams who have to send multiple payments/delegations are doing this work manually most of the times.

Proposal

Option-1: Add a flag --multi or --multiple for each txn, which will take either comma separated list of arguments to generate multiple messages.
Ex:

simd tx bank send to_addr1 amount1 --multi to_addr2,amount2,to_addr3,amount3,... --from from_addr

Option-2: Add a flag --multi or --multiple which takes a csv input for each txn and generates multiple messages.
Ex:

simd tx bank send to_addr1 amount1 --multi multi.csv --from from_addr

Where multi.csv would have the list of to_address, amount fields for MsgSend message. Similarly it will have validator_addr, delegation_amount for delegation transaction.

[alexanderbez]

Wouldn't it be better to generalize this? Meaning, we already have a means to generate unsigned txs. A user would generate 2 or more unsigned txs and then take those files and supply them to a command that constructs a signed multi-msg tx.

E.g.

$ simd tx bank send <args...|flags...> --generate-only > tx1.json
$ simd tx staking delegate <args...|flags...> --generate-only > tx2.json
...
$ simd tx multi-msg tx1.json tx2.json <args...|flags...> > tx.json

[anilcse]

That sounds great, like it. We can also combine both solutions if others see value in both options.