add SCP support

SCP is an experimental feature, we have our own SCP implementation
since we can't rely on scp system command to proper handle permissions,
quota and user's home dir restrictions. The SCP protocol is quite simple
but there is no official docs about it, so we need more testing and
feedbacks before enabling it by default.
We may not handle some borderline cases or have sneaky bugs.

This commit contains some breaking changes to the REST API.
SFTPGo API should be stable now and I hope no more breaking changes
before the first stable release.

api/router.go

@@ -40,7 +40,7 @@ func initializeRouter() {
 	})
 
 	router.Delete(activeConnectionsPath+"/{connectionID}", func(w http.ResponseWriter, r *http.Request) {
-		handleCloseSFTPConnection(w, r)
+		handleCloseConnection(w, r)
 	})
 
 	router.Get(quotaScanPath, func(w http.ResponseWriter, r *http.Request) {
@@ -72,7 +72,7 @@ func initializeRouter() {
 	})
 }
 
-func handleCloseSFTPConnection(w http.ResponseWriter, r *http.Request) {
+func handleCloseConnection(w http.ResponseWriter, r *http.Request) {
 	connectionID := chi.URLParam(r, "connectionID")
 	if connectionID == "" {
 		sendAPIResponse(w, r, nil, "connectionID is mandatory", http.StatusBadRequest)