[MM-25647]: ability to permanently delete users (#14944)

Summary:
    Config option to allow permanent user deletion ServiceSettings.EnableAPIUserDeletion
    Expose permanent user deletion through API
    Local mode for delete user for use in mmctl

Ticket Link:
    Server part of https://mattermost.atlassian.net/browse/MM-25647

model/client4.go

@@ -1240,6 +1240,16 @@ func (c *Client4) DeleteUser(userId string) (bool, *Response) {
 	return CheckStatusOK(r), BuildResponse(r)
 }
 
+// PermanentDeleteUser deletes a user in the system based on the provided user id string.
+func (c *Client4) PermanentDeleteUser(userId string) (bool, *Response) {
+	r, err := c.DoApiDelete(c.GetUserRoute(userId) + "?permanent=" + c.boolString(true))
+	if err != nil {
+		return false, BuildErrorResponse(r, err)
+	}
+	defer closeBody(r)
+	return CheckStatusOK(r), BuildResponse(r)
+}
+
 // ConvertUserToBot converts a user to a bot user.
 func (c *Client4) ConvertUserToBot(userId string) (*Bot, *Response) {
 	r, err := c.DoApiPost(c.GetUserRoute(userId)+"/convert_to_bot", "")