Link / Symlink support

[Xuanwo]

fs

fs has native support for link and symlink

oss

OSS has Symlink API:

• PutSymlink
• GetSymlink

s3

S3 doesn't support this feature, but maybe we can use user metadata?

• S3 has system defined metadata: x-amz-website-redirect-location
  • Note the limitations: https://stackoverflow.com/questions/35042316/amazon-s3-multiple-keys-to-one-object#comment57863171_35043462
• S3 support user metadata, we can add a metadata like x-amz-meta-symlink

[Xuanwo]

If we introduce new user metadata, the user has to read/write data through go-storage, it's not expected.

Or, maybe we need to provide the ability for users? Add a new option that the user can enable this feature.

We can add an option like enable-simulated-link for storage services that don't have native symlink support.

[Xuanwo]

Fixed in #86