How to implement cachebusting / versioning for an external file called within a template

At times I call external .css and .js within templates. I would like to able to use XenForo's built in cache busting for this. Without referencing PHP filemtime directly within a template what is the best way to go about this?

Sample css reference within template:

Code:

<link rel="stylesheet" href="https://www.mysite.com/styles/custom.css" type="text/css" />

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