feat: added "fraction" statistic to the "analyze_var_count" method group

[kpagacz]

Hello,

I wanted to display the count statistic in roughly this format: xx / xx (xx.xx%), but it's impossible to use analyze_vars in this manner. I could implement my own analyze_vars and the associated functions, but I only need this slight change in the code, and copying the whole thing seems like an overkill.

Let me know if there's a more straightforward solution to display these statistics using the existing API (and not lose the S3 dispatch on different types of input variables).

@shajoezhu @pawelru @gogonzo If you can point me to the correct person to approach regarding this, I'd be grateful.

[github-actions]

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[kpagacz]

I have read the CLA Document and I hereby sign the CLA

[pawelru]

@kpagacz please add NEWS entry and fix the tests. Please ignore CLA bot for a while - it's a known issue.

@insightsengineering/nest-sme please kindly help with the review of this

[Melkiades]

lgtm! As mentioned, add news and fix tests. Thanks @kpagacz!!