Add gomod-sync

[andrerfcsantos]

Part of #1881

This PR introduces a new tool to the repo: gomod-sync. This is a little program to:

• List go.mod files and the Go versions they are declaring via the gomod-sync list command.
• Check if all go.mod files from the exercises are set to a specific version, with the gomod-sync check command. The desired version can be set with the --goversion flag.
• Update all go.mod files to a specific version with the gomod-sync update command. The desired version for the update can be set with the --goversion flag.

Right now these 3 commands are working. The update command was ran against the repo to update all exercises to 1.16, and the result is #1955

Tasks

• list, check and update commands
• Setup the use of the check command in CI
• Documentation for the tool
• Add a way to allow specific go versions for specific exercises
• Tests

[github-actions]

Dear andrerfcsantos

Thank you for contributing to the Go track on Exercism! 💙
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[andrerfcsantos]

Ready for review 🚀

This one is on the bigger side, so feel free to take your time, there is no rush.

This is what the output of the check looks like when it fails: https://github.com/exercism/go/runs/4662874598?check_suite_focus=true

For the output in case everything is fine, see the go.mod check below ⬇️

[junedev]

I tried to do a review. I don't know anything about cobra though so I can't comment on that. The part that I could understand looked good to me, just some minor comments.

[andrerfcsantos]

@junedev Thanks! Applied all the changes. Left comments in the conversations, feel free to un-resolve any of them or suggest further changes if there's something that can be further improved.

[junedev]

@andrerfcsantos Thanks for addressing all the comments. Feel free to merge whenever you like. (I don't think there will be more reviews if we wait longer tbh.)