s3: add Impossible Cloud as a new S3 provider

backend/s3/provider/ImpossibleCloud.yaml

@@ -0,0 +1,21 @@
+name: ImpossibleCloud
+description: Impossible Cloud Object Storage
+region:
+  eu-central-2: Frankfurt, Germany
+  eu-west-1: Amsterdam, Netherlands
+  eu-west-2: London, UK
+  eu-west-3: Paris, France
+  eu-east-1: Poznań, Poland
+  eu-north-1: Copenhagen, Denmark
+  us-east-1: New York, USA
+endpoint:
+  eu-central-2.storage.impossibleapi.net: Frankfurt, Germany
+  eu-west-1.storage.impossibleapi.net: Amsterdam, Netherlands
+  eu-west-2.storage.impossibleapi.net: London, UK
+  eu-west-3.storage.impossibleapi.net: Paris, France
+  eu-east-1.storage.impossibleapi.net: Poznań, Poland
+  eu-north-1.storage.impossibleapi.net: Copenhagen, Denmark
+  us-east-1.storage.impossibleapi.net: New York, USA
+location_constraint: {}
+acl: {}
+bucket_acl: true

backend/s3/s3_internal_test.go

@@ -44,6 +44,13 @@ func md5sum(t *testing.T, s string) string {
 }
 
 func (f *Fs) InternalTestMetadata(t *testing.T) {
+	// Impossible Cloud silently drops cache-control,
+	// content-disposition, content-encoding and content-language
+	// system metadata on retrieval (and thus also breaks the
+	// Content-Encoding response header that --s3-decompress relies on).
+	if f.opt.Provider == "ImpossibleCloud" {
+		t.Skip("Impossible Cloud does not preserve all S3 system metadata")
+	}
 	ctx := context.Background()
 	original := random.String(1000)
 	contents := gz(t, original)