Add machine readable key generation

[kelltrick]

Add an output option to enable machine readable key generation.

Command: dotnet monitor generatekey --output MachineJson
Output:

{
    "AuthorizationHeader": "Bearer eyJhbGciOiJFUzM4NCIsInR5cCI6IkpXVCJ9.eyJhdWQiOiJodHRwczovL2dpdGh1Yi5jb20vZG90bmV0L2RvdG5ldC1tb25pdG9yIiwiaXNzIjoiaHR0cHM6Ly9naXRodWIuY29tL2RvdG5ldC9kb3RuZXQtbW9uaXRvci9nZW5lcmF0ZWtleStNb25pdG9yQXBpS2V5Iiwic3ViIjoiZmEzZjVhYjQtMTUxOS00YmU3LTk1ZTEtYTRhYjYyMzQzYTIyIn0.L2EcOE4fsVXKcWQW2IIOUvEX8g5TLj4-UxeCTSknvYeYROUxkU3iJ0u94vacpvbfDPG-UlxnvdRU_w-qA2xN7bDUS-nkK5eHnccfy8DCLs9Afm048IB2ZXJVu4UrFUMp",
    "Authentication": {
        "MonitorApiKey": {
          "Subject": "fa3f5ab4-1519-4be7-95e1-a4ab62343a22",
          "PublicKey": "eyJBZGRpdGlvbmFsRGF0YSI6e30sIkNydiI6IlAtMzg0IiwiS2V5T3BzIjpbXSwiS3R5IjoiRUMiLCJYIjoiOGZyTEhoSHlwTkx6SEhOMXdiZUpWeVJzeDR6cmNTYzdhMWFKcjNMUm1lMDVXVlh0MTVGMk85YTlzUEVYUlpIVSIsIlg1YyI6W10sIlkiOiJUUU53SWU0Sksyci1JTXc3RURYTjJMR0ZQTTZiV29QeTZDMmVqTXllRkNCT0NIZVFTOGZTQlZTZ1ZweUt4U1ZwIiwiS2V5U2l6ZSI6Mzg0LCJIYXNQcml2YXRlS2V5IjpmYWxzZSwiQ3J5cHRvUHJvdmlkZXJGYWN0b3J5Ijp7IkNyeXB0b1Byb3ZpZGVyQ2FjaGUiOnt9LCJDYWNoZVNpZ25hdHVyZVByb3ZpZGVycyI6dHJ1ZSwiU2lnbmF0dXJlUHJvdmlkZXJPYmplY3RQb29sQ2FjaGVTaXplIjozMn19"
        }
    }
}