Allows creating new remote clusters without providing a password (#27864)

* Allows creating new remote clusters without providing a password

If the endpoint receives a request with no password, it will generate
one internally and return it in the response, so the frotend can show
it to the user.

* Use a random string instead of a UUID for the generated password

* Update function name to avoid CString reference and adds assertion

* Update server/channels/utils/textgeneration.go

Co-authored-by: Eva Sarafianou <eva.sarafianou@gmail.com>

* Extends the charset

---------

Co-authored-by: Eva Sarafianou <eva.sarafianou@gmail.com>

server/channels/api4/remote_cluster.go

@@ -13,6 +13,7 @@ import (
 	"github.com/mattermost/mattermost/server/public/shared/mlog"
 	"github.com/mattermost/mattermost/server/v8/channels/app"
 	"github.com/mattermost/mattermost/server/v8/channels/audit"
+	"github.com/mattermost/mattermost/server/v8/channels/utils"
 	"github.com/mattermost/mattermost/server/v8/platform/services/remotecluster"
 )
 
@@ -366,11 +367,6 @@ func createRemoteCluster(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if rcWithTeamAndPassword.Password == "" {
-		c.SetInvalidParam("password")
-		return
-	}
-
 	url := c.App.GetSiteURL()
 	if url == "" {
 		c.Err = model.NewAppError("createRemoteCluster", "api.get_site_url_error", nil, "", http.StatusUnprocessableEntity)
@@ -398,7 +394,12 @@ func createRemoteCluster(c *Context, w http.ResponseWriter, r *http.Request) {
 	}
 	rcSaved.Sanitize()
 
-	inviteCode, iErr := c.App.CreateRemoteClusterInvite(rcSaved.RemoteId, url, rcSaved.Token, rcWithTeamAndPassword.Password)
+	password := rcWithTeamAndPassword.Password
+	if password == "" {
+		password = utils.SecureRandString(16)
+	}
+
+	inviteCode, iErr := c.App.CreateRemoteClusterInvite(rcSaved.RemoteId, url, rcSaved.Token, password)
 	if iErr != nil {
 		c.Err = iErr
 		return
@@ -408,7 +409,12 @@ func createRemoteCluster(c *Context, w http.ResponseWriter, r *http.Request) {
 	auditRec.AddEventResultState(rcSaved)
 	auditRec.AddEventObjectType("remotecluster")
 
-	b, err := json.Marshal(model.RemoteClusterWithInvite{RemoteCluster: rcSaved, Invite: inviteCode})
+	resp := model.RemoteClusterWithInvite{RemoteCluster: rcSaved, Invite: inviteCode}
+	if rcWithTeamAndPassword.Password == "" {
+		resp.Password = password
+	}
+
+	b, err := json.Marshal(resp)
 	if err != nil {
 		c.Err = model.NewAppError("createRemoteCluster", "api.marshal_error", nil, "", http.StatusInternalServerError).Wrap(err)
 		return