feat: add uuid widget

[liufuyang]

Add a uuid widget which supports options:

• read_only (bool)
• prefix (string)
• use_b32_encoding (bool)

close: https://github.com/netlify/netlify-cms/issues/1975

Summary

Test plan

Checklist

Please add a x inside each checkbox:

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A picture of a cute animal (not mandatory but encouraged)

So we can know easily having read-only UUID field like this:

And when prefix as "sink/" and use_b32_encode is used, it shows up as this:

[dopry]

@liufuyang do you have time to update this pull request?

[liufuyang]

Thank you, I can try to find some time recently. Perhaps just rebase onto master will do?

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[martinjagodic]

@liufuyang I know it's been a while, would you be able to brig the branch up to date with master and solve merge conflicts? Thanks!