Add support for new Steam authentication

[yaakov-h]

No description provided.

[xPaw]

Can probably drop SentryFileHash/accepting machine auth with this.

[yaakov-h]

I haven't had a chance to go over it with a fine-toothed comb but this seems to be working well enough, if it breaks then I'm happy to take bug reports as they come in.

[xPaw]

Codes I've seen are InvalidSignature and AccessDenied.

InvalidPassword probably happens when sending password in logon.

[ABaumher]

I probably don't belong here, but while i was testing your code, i noticed a minor error at

DepotDownloader/DepotDownloader/Steam3Session.cs

Line 679 in 0112ca7

Console.WriteLine("Got Machine Auth: {0} {1} {2} {3}", machineAuth.FileName, machineAuth.Offset, machineAuth.BytesToWrite, machineAuth.Data.Length, hash);

You provide 5 arguments to string.format via Console.WriteLine, the formatted string only has 4. Can be fixed by adding {4}.
Console.WriteLine("Got Machine Auth: {0} {1} {2} {3} {4}", machineAuth.FileName, machineAuth.Offset, machineAuth.BytesToWrite, machineAuth.Data.Length, hash);

Edit: clarified ConsoleWriteline, not string.format

[xPaw]

@ABaumher fixed that in 0e8b662.

I merged this PR so it can be tested by users, and improvements can be made in further PRs.

[GoTeamScotch]

Just to clarify, does this allow the use of -remember-password (referenced by #397)?

Trying out v2.5.0 tonight and I seem to be getting multiple Steam Guard verification code requests.

Example:

DepotDownloader.exe -app 223100 -depot 223101 -username GoTeamScotch -remember-password
DepotDownloader.exe -app 223100 -depot 223102 -username GoTeamScotch

leads to multiple requests for the account password, then a subsequent Steam Guard code via email.