feat: specify owner index on coinbase smart wallet

[stephancill]

Adds an ownerIndex to toCoinbaseSmartWallet params which allows the user specify the index of the owner to be encoded in signatures. Discussion: #2851

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PR-Codex overview

This PR introduces the ownerIndex parameter to the toCoinbaseSmartAccount function, allowing users to specify which owner to use for signing messages. It includes updates to the documentation and tests to reflect this new functionality.

Detailed summary

• Added ownerIndex parameter to toCoinbaseSmartAccount.
• Updated documentation to include ownerIndex details.
• Modified toCoinbaseSmartAccount implementation to utilize ownerIndex.
• Enhanced tests to cover scenarios with multiple owners and different ownerIndex values.

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🦋 Changeset detected

Latest commit: 71a19d7

The changes in this PR will be included in the next version bump.

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