Add helper functions to inspect and create key packets

[lubux]

Adds the following API functions:

• API to get signature key IDs for mobile:

   func (msg *PGPMessage) GetHexSignatureKeyIDsJson() []byte

• API to get encryption key IDs for mobile:

   func (msg *PGPMessage) GetHexEncryptionKeyIDsJson() []byte

• API to get the number of key packets in a PGP message:

   func (msg *PGPSplitMessage) GetNumberOfKeyPackets() (int, error)

• API in package helper to encrypt a PGP message to an additional key:

   func EncryptPGPMessageToAdditionalKey(messageToModify *crypto.PGPSplitMessage, keyRing *crypto.KeyRing, additionalKey *crypto.KeyRing) error