Make IMAP modular

As an IMAP administrator I want to customize IMAP processing easily. This includes removing/adding/changing IMAP processors/encoders/decoders.

This allows the following use cases:

• Add support for (potentially custom) IMAP extensions without recompiling James
• Removing support for buggy commands easily, if any
• Enforcing "read only behaviour"
• Customizing some commands. For instance one of my customers got a tricky non standard IMAP authenticate command to handle.

The admin would specify a list of ImapPackage (which is itself a bundle of processors/encoders/decoders) from the imapserver.xml configuration file. By default, if unspecified the default IMAP package would be used.

Both ImapPackage/processors/encoders/decoders loading would leverage the extensions-jars mechanism.

Definition of done

Write an exemple of such modularization writing a PING / PONG command and load it into a James server.

A001 PING whatever
 * PONG whatever
A001 OK PING command completed.