Add new type to represent validating required flag error

[sivchari]

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Currently, when we met the ValidateRequiredField error, we should compare as a raw string. This proposal aims to compare by using errors.Is/As and csting from interface to the type.

[CLAassistant]

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[eth-p]

This feature was also implemented by PR #2266, and that PR also includes an error type added by an older PR, #2178 (CC @mvo5).

Seeing as how we have three different PRs aiming to achieve the same goal of making it possible to determine the type of and unwrap errors generated by Cobra, I think it would be a good idea to focus our combined efforts around a single implementation.

I might be a bit biased as the author of #2266, but that one covers pretty much every user-caused error type that Cobra can return. For what it's worth mentioning as well, a similar effort was made to add error structs to spf13/pflag. It would be great to have cobra fully support them as well.

[ccoVeille]

Thanks for cross referencing things.

I saw the previous one @eth-p

Then I saw this one, I thought it was from the same author, that had closed then reopened a new PR

I have already shared my concerns with the implementation in this PR