Support different digests (SHA-256, SHA-512) in temporary URLs

[AlexandrePhilibert]

The current implementation of ObjectTempUrl only supports SHA-1, but the temporary url middleware in swift can support SHA-256 and SHA-512.

I think the best way to add support for different digests would be by introducing a new parameter to ObjectTempUrl, but this would be a breaking change.

[ncw]

I'd prefer not to break the API

The current function looks like this

ObjectTempUrl(container string, objectName string, secretKey string, method string, expires time.Time) string

How about introducing ObjectTempUrlShaA256 and ObjectTempUrlSha512 and then factoring the current ObjectTempUrl into a private utility function which takes the hash contstructor as an argument?