token-js: Properly support `OptionalNonZeroPubkey`

[joncinque]

Problem

In order to pack things efficiently and properly use the bytemuck Zeroable and Pod traits, the extensions use OptionalNonZeroPubkey, which maps to None if the pubkey is all zeroes, and Some(pubkey) for all other values. Unfortunately, these are all currently mapped as PublicKey in token-js.

Solution

Map these types to PublicKey? with the proper conversions.