Add JSON output for download_single

[DrChat]

This allows us to use this command entirely programmatically, easing its integration into our automation.

Example

     Running `target\debug\pdblister.exe download_single SRV*C:\Symbols*https://msdl.microsoft.com/download/symbols C:\Windows\System32\notepad.exe --message-format=json`
{"message":"file already cached","path":"C:\\Symbols\\notepad.pdb\\C694C0AA7279CC672966901283BF50541\\notepad.pdb","status":"success"}