Revert new SiaPublicKey JSON encoding

!3199 (merged) broke API compatibility by changing how public keys are encoded. I'm not sure why I thought this was ok. I mentioned in that MR description that it wasn't a big deal because JSON isn't sent over the network. But JSON is sent over the API! Ugh.

We'll keep the new flexible UnmarshalJSON method, so users can still use the new encoding when e.g. manually constructing transactions (which is my desired use-case).