Add flutter_cached_pdfview in pdf plugins section

[binSaed]

Description

create pdf section and add my awesome package flutter_cached_pdfview

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Checklist

• [x] I read How to contribute
• [x] I edited the SOURCE.md file only

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Feel free to add this badge to your repository after it's accepted to awesome-flutter.

• [x] I've added it to my repo

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