feat: s3 driver support for ExistsPath

Related to #752 (closed)

JH SaaS is deployed on Tencent Cloud, the container registry uses the cloud object storage (COS, S3 compatible) as its storage driver, but it doesn't support S3 storage when container registry enable metadata database feature.

So this mr is implementation of S3 storage support when container registry enable metadata database feature.

I implemented ExistsPath method used for gradual migration mode and the related import API paths.

/cc @prajnamas