Ensure Ledger users can sign Agoric transactions

[rowgraus]

As an Agoric user who manages my assets with a hardware wallet (Ledger), I need to be able to sign transactions with Agoric offers (e.g., create a vault, trade on AMM, add liquidity on AMM) using my hardware wallet.

Our on-chain wallet design may handle this natively through Keplr. This should be explicitly tested.

Must have:

• User can sign a transaction hash
• Ledger support

Nice/prefer to have:

• Offer give and want is clearly displayed to user
• Ledger support

[Tartuffo]

@rowgraus What is the open question here?

[rowgraus]

Story updated. Assigned to Michael. Should be estimated as non-zero now @Tartuffo @michaelfig

[michaelfig]

Using a Ledger to sign wallet spend transactions (including offers) should require only minimal testing, as Keplr makes that process transparent to our software. This issue entails working through the main wallet flows using a hardware wallet supported by Keplr.

I consider the nice-to-haves to be out of scope for this issue. Trezor is not supported by Keplr, and for now, Keplr is our only supported signer. It would be great if there are other signers available (MetaMask snaps and Cosmostation come to mind), but integrating and testing each one of them should be a separate issue, ideally not waiting on the Agoric team.

Offer legibility on hardware wallets will require coordination with Keplr around SIGN_MODE_TEXTUAL (#4529). There will likely be broader Cosmos ecosystem support for Ledger+SIGN_MODE_TEXTUAL, and integrating or driving that will require significant resources, so I'm considering it out of scope for Mainnet 1.

If any of these nice-to-haves become necessary for Mainnet 1, let's create separate issues.