RDP-specific Coder Desktop deep link

[ThomasK33]

Add RDP-specific logic into Coder Desktop so that all coder apps marked with external=true and a URL matching to coder:/v0/open/agent/<agent ID>/rdp?username=<username>&password=<passwpord> will be rendered as buttons and have RDP-specific logic in place. This logic can include fetching additional information or credentials in Coder Desktop and creating an .rdp file or comparable integrations.