Add --sig and --no-sig flag for RBS signatures

[packrat386]

What was the end-user or developer problem that led to this PR?

I do not tend to use RBS signatures, so when I create a new gem I would like the option to not generate them in the first place.

What is your fix for the problem, implemented in this PR?

This adds --sig and --no-sig so the user can specify whether or not they want signatures. The default is enabled so nothing changes for anybody used the existing behavior unless they opt in.

I understand based on discussion in #5041 that this was deemed unnecessary (specifically #5041 (comment)), but I think that after a few years of having RBS signatures available it seems that usage is not so universal as to justify having no opt out (besides deleting the files). I didn't add any persistent config to make --no-sig a system default since it seems like we probably like having the nudge towards using RBS signatures by default, but at least in my own use of bundle gem I would appreciate being able to turn it off.

Make sure the following tasks are checked

• Describe the problem / feature
• Write tests for features and bug fixes
• Write code to solve the problem
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