Add HIP-765: Non-Fungible Token Metadata Field

[RickyLB]

Problem

HIP: hips.hedera.com/hip/hip-765

Protobuf PR: hashgraph/hedera-protobufs#313

This HIP proposes the addition of the metadata field to Non-Fungible Token Class, taking after the individual Non-Fungible Token (NFT) metadata field which was added in HIP-17. While HIP-17 metadata is scoped to a single NFT, here we propose adding metadata scoped to an entire token class.

Solution

Implement the HIP
References Issue: hashgraph/hedera-sdk-reference#148

Alternatives

No response