Add MCOIMAPConnectOperation to get server capabilities without logging in.

It's often useful to know what default capabilities an IMAP server supports. For example, a server may support STARTTLS over port 143, or advertise which authentication types it supports in its welcome message.

This patch adds an MCOIMAPConnectOperation, which simply connects to the server without logging in. That way, information about authentication and connection types can be read into the IMAP session capabilities and then a separate login operation can be performed using MCOIMAPCheckAccountOperation.

I've also added STARTTLS and LOGIN/PLAIN checks to the server capabilities parser in MCIMAPAsyncSession.cc.