refactor(client/v2): refactor of flags

[julienrbrt]

Description

Extracting some refactor from #16985, for improving code readability.

Follow-up PRs will add support for repeated coins and actually setting the messages and usagage of keyring interface.

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Author Checklist

All items are required. Please add a note to the item if the item is not applicable and
please add links to any relevant follow up issues.

I have...

• included the correct type prefix in the PR title
• added ! to the type prefix if API or client breaking change
• targeted the correct branch (see PR Targeting)
• provided a link to the relevant issue or specification
• followed the guidelines for building modules
• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
• included comments for documenting Go code
• updated the relevant documentation or specification
• reviewed "Files changed" and left comments if necessary
• run make lint and make test
• confirmed all CI checks have passed

Reviewers Checklist

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I have...

• confirmed the correct type prefix in the PR title
• confirmed ! in the type prefix if API or client breaking change
• confirmed all author checklist items have been addressed
• reviewed state machine logic
• reviewed API design and naming
• reviewed documentation is accurate
• reviewed tests and test coverage
• manually tested (if applicable)

[tac0turtle]

utACK