Add Class 2 fax protocol support

- class2/commands.rs: EIA/TIA-592 Class 2 AT command definitions
- class2/send.rs: Class2Send implementation for simpler fax sending
- CLI: Add --class option for send command (1 or 2)
- CLI: Add --class2 flag for test command
- Tested with Conexant V90 modem - all Class 2 commands working

src/fax/mod.rs

@@ -2,6 +2,8 @@ pub mod t4;
 pub mod hdlc;
 pub mod negotiate;
 pub mod class1;
+pub mod class2;
 
 pub use class1::Class1Session;
+pub use class2::Class2Send;
 pub use t4::T4Codec;