Package extract provides a shared token/value extractor for middleware that
reads values from request headers, query parameters, cookies, form fields,
or path parameters using a "source:name[:prefix]" lookup string format.
Parse parses a comma-separated lookup string into an extractor function.
Format: "source:name[:prefix]" where source is header, query, cookie, form, or param.
Multiple sources separated by commas are tried in order; the first non-empty result wins.