[Code] disk watermark supports percentage and absolute modes

[mw-ding]

Summary

Support percentage and absolute mode for specifying disk watermark configs in Code to keep align with Elasticsearch here

Percentage Mode Config:

disk: {
  thresholdEnabled: true,
  watermarkLow: "90%",
}

server will complain when disk usage is above 90%.

Absolute Mode Config:

disk: {
  thresholdEnabled: true,
  watermarkLow: "100mb",
}

server will complain when available/free disk space left is below 100mb. For now, only kb, mb, gb, tb are supported.

By default, the percentage mode with 90% is used. @chongzhe we might need to update the docs.

Screenshots
Percentage mode:

Absolute mode:

Checklist

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• This was checked for cross-browser compatibility, including a check against IE11
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• Unit or functional tests were updated or added to match the most common scenarios
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For maintainers

• This was checked for breaking API changes and was labeled appropriately
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