Implemented XOAUTH2 authentication for POP3

[jstanden]

Authentication with XOAUTH2 tokens was previously implemented for the IMAP protocol but not for POP3.

This PR implements the auth method for POP3 accounts.

Gmail supports XOAUTH2 tokens for both protocols:

$ openssl s_client -connect pop.gmail.com:995 -crlf

+OK Gpop ready for requests from 192.168.1.1 a1b2c3d4e5
capa
+OK Capability list follows
USER
RESP-CODES
EXPIRE 0
LOGIN-DELAY 300
TOP
UIDL
X-GOOGLE-RICO
SASL PLAIN XOAUTH2 OAUTHBEARER

This uses the same xoauth2_token parameter as IMAP, and the same Horde_Imap_Client_Password_Xoauth2 class.

In POP3, the command is auth xoauth2 <base64-encoded auth-bearer>.

This patch also moves XOAUTH2 to the first attempted authentication mechanism if the xoauth2_token parameter is set; which makes USER with an app-specific password the optional fallback.

[what-the-diff]

PR Summary

• Enhanced Authentication Method
  We've added a feature where the system will always use XOAUTH2 (a secure method for authentication) when a token (kind of like an access key) is provided.

• Updated Login System
  The part of the program that handles the login operation has been upgraded to manage the XOAUTH2 process, validating the presence of the token before proceeding.

• Debug Information for Enhanced Authentication
  This update also includes the ability to track the authentication process, logging the user related to the token for debugging and error-tracking purposes.

[TDannhauer]

Looks good for me.

[TDannhauer]

If Horde_Imap_Client_Password_Xoauth2 is now used for IMAP as well as POP3, shouldn't the class be renamed/relocated to avoid confusion and fit poperly into the structure?

[jstanden]

@TDannhauer Hello Torben! I'm not sure I follow.

The library is named Horde_Imap, so all of the existing classes (including POP3) are already prefixed:

• Horde_Imap_Client_Socket_Connection_Pop3
• Horde_Imap_Client_Socket_Pop3
• Horde_Imap_Client_Ids_Pop3

The password classes are:

• Horde_Imap_Client_Base_Password
• Horde_Imap_Client_Password_Xoauth2

That feels consistent. The directory structure follows the class naming convention.

I do need to fix an XOAUTH2 protocol issue between Microsoft POP3 and Gmail POP3, though. The implementation uses a single line AUTH statement, but Microsoft only supports the two-line style (with the token on a second line).

[TDannhauer]

sorry missed that. Fine for me.

[jstanden]

It looks like Microsoft POP3 requires a two-line AUTH XOAUTH command, with the token on the second line.

Gmail POP3 is fine with either one-line or two-line.

I'll update the PR to always use the two-line format.

[jstanden]

I updated the commit to always use the two-line AUTH XOAUTH2 format.