Add checksum to "bundle" field of plugin package.json

[danfinlay]

Along with adding more decentralized distribution channels, having the checksum encoded would ensure we could load plugins with integrity.

[rekmarks]

This has been done as of the v0.1 snaps manifest: https://github.com/MetaMask/specifications/blob/d4a5bf5d6990bb5b02a98bd3f95a24ffb28c701c/snaps/publishing.md