Adds support for Cloudflare R2 Storage

[emilianscheel]

Thank you for providing such a convenient way to implement additional object storage providers! This pull request is supposed to add a fully-functional integration of Cloudflare R2 and Sanity Plugin External Files which includes ...

• Uploading/ deleting capabilities
• Documentation for configuring plugin/ cloudflare located in README.md
• Cloudflare Worker script including deployment configuration

Example Usage