Add a constructor to create an OlmPkDecryption object from a byte array
The OlmPkDecryption object is used to construct what the spec1 calls a Recovery key. This key can be backed up as a base 58 encoded string, restoring the backup reverses the process. We get a byte array when we decode the base 58 representation of the key.
This patch adds support to construct a OlmPkDecryption object from such a byte array.