Add store listing metadata upload support #23

Add support for uploading store listing metadata (screenshots, icons, descriptions, videos) to Amazon Appstore.

• POST /listings/{language}/{imageType}/upload - Upload images (screenshots, icons, promo images)
• POST /listings/{language}/videos - Upload videos
• PUT /listings/{language} - Update store listing text (title, descriptions)

References

https://amzndevresources.com/appsubapi/swagger-en/index.html