s3: add --s3-sts-endpoint to specify STS endpoint

See: https://forum.rclone.org/t/s3-profile-failing-when-explicit-s3-endpoint-is-present/36063/

backend/s3/s3.go

@@ -2266,6 +2266,11 @@ rclone's choice here.
 			Help:     `Suppress setting and reading of system metadata`,
 			Advanced: true,
 			Default:  false,
+		}, {
+			Name:     "sts_endpoint",
+			Help:     "Endpoint for STS.\n\nLeave blank if using AWS to use the default endpoint for the region.",
+			Provider: "AWS",
+			Advanced: true,
 		},
 		}})
 }
@@ -2352,6 +2357,7 @@ type Options struct {
 	SecretAccessKey       string               `config:"secret_access_key"`
 	Region                string               `config:"region"`
 	Endpoint              string               `config:"endpoint"`
+	STSEndpoint           string               `config:"sts_endpoint"`
 	LocationConstraint    string               `config:"location_constraint"`
 	ACL                   string               `config:"acl"`
 	BucketACL             string               `config:"bucket_acl"`
@@ -2566,16 +2572,24 @@ var defaultResolver = endpoints.DefaultResolver()
 // resolve (service, region) to endpoint
 //
 // Used to set endpoint for s3 services and not for other services
-type resolver string
+type resolver map[string]string
+
+// Add a service to the resolver, ignoring empty urls
+func (r resolver) addService(service, url string) {
+	if url == "" {
+		return
+	}
+	if !strings.HasPrefix(url, "http") {
+		url = "https://" + url
+	}
+	r[service] = url
+}
 
 // EndpointFor return the endpoint for s3 if set or the default if not
-func (endpoint resolver) EndpointFor(service, region string, opts ...func(*endpoints.Options)) (endpoints.ResolvedEndpoint, error) {
+func (r resolver) EndpointFor(service, region string, opts ...func(*endpoints.Options)) (endpoints.ResolvedEndpoint, error) {
 	fs.Debugf(nil, "Resolving service %q region %q", service, region)
-	if service == "s3" {
-		url := string(endpoint)
-		if !strings.HasPrefix(url, "http") {
-			url = "https://" + url
-		}
+	url, ok := r[service]
+	if ok {
 		return endpoints.ResolvedEndpoint{
 			URL:           url,
 			SigningRegion: region,
@@ -2662,9 +2676,12 @@ func s3Connection(ctx context.Context, opt *Options, client *http.Client) (*s3.S
 	if opt.Region != "" {
 		awsConfig.WithRegion(opt.Region)
 	}
-	if opt.Endpoint != "" {
-		// If endpoint is set, only override the s3 service so we don't break sts
-		awsConfig.WithEndpointResolver(resolver(opt.Endpoint))
+	if opt.Endpoint != "" || opt.STSEndpoint != "" {
+		// If endpoints are set, override the relevant services only
+		r := make(resolver)
+		r.addService("s3", opt.Endpoint)
+		r.addService("sts", opt.STSEndpoint)
+		awsConfig.WithEndpointResolver(r)
 	}
 
 	// awsConfig.WithLogLevel(aws.LogDebugWithSigning)