Add serde for EncryptedBlindIndexSalt

[BobWall23]

As a developer consuming the ironoxide search functionality, I need to be able to save the output of the create_blind_index call, a EncryptedBlindIndexSalt, and I need to be able to recreate that object to subsequently initialize the index.

EncryptedBlindIndexSalt should provide a way to easily serialize from and deserialize to an instance. We should use serde, to give the implementer the choice of format for serialized version.