feat: add response header overrides for GetObject

GetObject allows overriding response headers with the following
paramters:
response-cache-control
response-content-disposition
response-content-encoding
response-content-language
response-content-type
response-expires

This is only valid for signed (and pre-singed) requests. An error
is returned for anonymous requests if these are set.

More info on the GetObject overrides can be found in the GetObject
API reference.

This also clarifies the naming of the AccessOptions IsPublicBucket
to IsPublicRequest to indicate this is a public access request
and not just accessing a bucket that allows public access.

Fixes #1501

s3err/s3err.go

@@ -64,6 +64,7 @@ const (
 	ErrAnonymousCopyObject
 	ErrAnonymousPutBucketOwnership
 	ErrAnonymousGetBucketOwnership
+	ErrAnonymousResponseHeaders
 	ErrMethodNotAllowed
 	ErrBucketNotEmpty
 	ErrVersionedBucketNotEmpty
@@ -225,6 +226,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "s3:GetBucketOwnershipControls does not support Anonymous requests!",
 		HTTPStatusCode: http.StatusForbidden,
 	},
+	ErrAnonymousResponseHeaders: {
+		Code:           "InvalidRequest",
+		Description:    "Request specific response headers cannot be used for anonymous GET requests.",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 	ErrMethodNotAllowed: {
 		Code:           "MethodNotAllowed",
 		Description:    "The specified method is not allowed against this resource.",