Implement OpenPGP Packet Criticality

[vanitasvitae]

The upcoming OpenPGP v6 specification introduces the concept of Packet Criticality based on packet tags.

This PR adds support for packet criticality by adding getPacketTag() to the Packet class.
Further, it introduces a dedicated UnknownPacket class to improve forwards compatibility with future revisions of the standard, as well as a dedicated ReservedPacket class.

[vanitasvitae]

OpenPGP v6 implementations would set pgpObjectFactory.setThrowForUnknownCriticalPackets(true);, such that unknown critical packets automatically cause an IOException to be thrown.
If this flag is not set, the object factory will instead return an UnknownPacket object which the user can further consume.

[dghgit]

Thanks for the patch. Merged with some changes (trying to preserve API compatibility). This is also now published on https://www.bouncycastle.org/betas 176b05 or later.