auth: Move stdsignmsg to txbuilder, add tests

[ValarDragon]

This is done because stdsignmsg is only used within the txbuilder. It explicitly states only for use within the CLI, hence it belongs in one of the client packages.

• Linked to github-issue with discussion and accepted design OR link to spec that describes this work. - sufficiently simple imo
• Wrote tests
• Updated relevant documentation (docs/) - anything I should be updating?
• Added entries in PENDING.md with issue #
• rereviewed Files changed in the github PR explorer

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For Admin Use:

• Added appropriate labels to PR (ex. wip, ready-for-review, docs)
• Reviewers Assigned
• Squashed all commits, uses message "Merge pull request #XYZ: [title]" (coding standards)

[alexanderbez]

utACK 👍

[cwgoes]

utACK, thanks for the testcases 🎉

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (develop@145e06b). Click here to learn what that means.
The diff coverage is 22.22%.

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