Create SIGN_MODE_AMINO_AUX for signing meta-transactions

[amaury1093]

Part of the TX working group

Summary

Create a new sign mode called SIGN_MODE_AMINO_AUX

Problem Definition

The meta-transactions feature (#9912) needs a SIGN_MODE for signing tips. We added SIGN_MODE_AUX, but ledger devices can't sign that.

Our first approach was to create a totally new SIGN_MODE for ledger: either PROTO_JSON or TEXTUAL. Unfortunately, the first one is not perfect for ledger, and the second one needs more specification work.

For the meta-tx feature to be shipped asap, we propose to create a SIGN_MODE_AMINO_AUX based on the existing StdSignDoc, and used to sign tipped txs using Amino (so compatible with Ledger). This would require minimal work on the SDK, on CosmJS/client-devs and on the Ledger app teams.

Proposal

1. Create a new StdSignDocAux:

type StdSignDocAux struct {
	AccountNumber uint64            `json:"account_number" yaml:"account_number"`
	Sequence      uint64            `json:"sequence" yaml:"sequence"`
	TimeoutHeight uint64            `json:"timeout_height,omitempty" yaml:"timeout_height"`
	ChainID       string            `json:"chain_id" yaml:"chain_id"`
	Memo          string            `json:"memo" yaml:"memo"`
	Msgs          []json.RawMessage `json:"msgs" yaml:"msgs"`
	// Remove `Fee` field from StdSignDoc, and add this
	Tip           []sdk.Coins // or []json.RawMessage? Look at output and fee if there's any difference between using sdk.Coins and []json.RawMessage
}

2. Create a new sign mode handler for SIGN_MODE_AMINO_AUX
3. Add tests

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