s3: Add config option to set storage class

The `s3.storage-class` option can be passed to restic (using `-o`) to
specify the storage class to be used for S3 objects created by restic.

The storage class is passed as-is to S3, so it needs to be understood by
the API. On AWS, it can be one of `STANDARD`, `STANDARD_IA`,
`ONEZONE_IA`, `INTELLIGENT_TIERING` and `REDUCED_REDUNDANCY`. If
unspecified, the default storage class is used (`STANDARD` on AWS).

You can mix storage classes in the same bucket, and the setting isn't
stored in the restic repository, so be sure to specify it with each
command that writes to S3.

Closes #706

internal/backend/s3/config.go

@@ -18,6 +18,7 @@ type Config struct {
 	Bucket        string
 	Prefix        string
 	Layout        string `option:"layout" help:"use this backend layout (default: auto-detect)"`
+	StorageClass  string `option:"storage-class" help:"set S3 storage class (STANDARD, STANDARD_IA, ONEZONE_IA, INTELLIGENT_TIERING or REDUCED_REDUNDANCY)"`
 
 	Connections uint `option:"connections" help:"set a limit for the number of concurrent connections (default: 5)"`
 	MaxRetries  uint `option:"retries" help:"set the number of retries attempted"`

internal/backend/s3/s3.go

@@ -260,7 +260,7 @@ func (be *Backend) Save(ctx context.Context, h restic.Handle, rd restic.RewindRe
 	be.sem.GetToken()
 	defer be.sem.ReleaseToken()
 
-	opts := minio.PutObjectOptions{}
+	opts := minio.PutObjectOptions{StorageClass: be.cfg.StorageClass}
 	opts.ContentType = "application/octet-stream"
 
 	debug.Log("PutObject(%v, %v, %v)", be.cfg.Bucket, objName, rd.Length())