Added xattr plugin for darwin.

[bmcdermott-r7]

Addressing issue #3273

[scudette]

Since code is really taking one value and returning one value it is probably better to do it as a VQL function rather than a plugin. Plugins are good for returning multiple rows but if we use a function it feels more natural to use it. For example instead of

As a plugin

SELECT * FROM foreach(row={ SELECT * FROM glob(....) }.
query={
  SELECT * FROM xattr(filename=OSPath)
})

As a function

SELECT *, xattr(filename=OSPath) AS Xattr FROM glob(...)

[CLAassistant]

All committers have signed the CLA.

[scudette]

Can you please not remove unrelated files? Also please dont change package-lock.json as it is managed by Snyke

[bmcder02]

Sorry AV removed it, and I didn't notice in the PR.