storage: implement soft delete feature

[JesseLovelace]

Please implement the soft delete feature as specified in go/gcs-soft-delete-client-request

The operations we need are

• [x] Set a soft delete policy on a new or existing bucket
• [x] Get a bucket's soft delete policy
• [x] Get the soft delete time and hard delete time metadata fields from a soft deleted object
• [x] Get a soft deleted object by doing an object get call with the soft deleted parameter set to true
• [x] List soft deleted objects by doing an object list call with the soft deleted parameter set to true
• [ ] Restore a soft deleted object

You can use the Java implementation as a reference