Add sigdump gem (Apache2 license)

What does this MR do and why?

Add sigdump gem, only for development, and test environments.

This tool was useful in debugging hung request specs (e.g. #337039 (comment 849225999))

No idea where is a suitable document to document this. Any suggestions ?

Usage: https://github.com/fluent/sigdump#usage

Related issue: #337039 (closed)

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How to set up and validate locally

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MR acceptance checklist

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