fs: allow setting of --http_proxy from command line

This in turn allows `override.http_proxy` to be set in backend configs
to set an http proxy for a single backend.

fs/config.go

@@ -555,6 +555,11 @@ var ConfigOptionsInfo = Options{{
 	Default: []string{},
 	Help:    "Transform paths during the copy process.",
 	Groups:  "Copy",
+}, {
+	Name:    "http_proxy",
+	Default: "",
+	Help:    "HTTP proxy URL.",
+	Groups:  "Networking",
 }}
 
 // ConfigInfo is filesystem config options
@@ -667,6 +672,7 @@ type ConfigInfo struct {
 	MetadataMapper             SpaceSepList      `config:"metadata_mapper"`
 	MaxConnections             int               `config:"max_connections"`
 	NameTransform              []string          `config:"name_transform"`
+	HTTPProxy                  string            `config:"http_proxy"`
 }
 
 func init() {