Feature: add an extra endpoint for s3 storage backend

[bitsf]

When we setup an internal s3 compatible storage backend and want to enable redirect for blobs pulling, registry distribution will use only one endpoint for pulling and pushing, that would make network for pushing. I hope to add a extra endpoint to separate the operation.
The setting can be added in config.yml

storage:
  s3:
     regionendpoint: https://192.168.0.100
     redirectendpoint: https://minio.harbor.xxx.com

or add in redirect section

storage:
  redirect:
     disable: false
     endpoint: https://minio.harbor.xxx.com
  s3:
     regionendpoint: https://192.168.0.100

[wy65701436]

We can bind these two issues? #3139