Setting PeerSharing Key Manually in config.json

In the current `config.json` file, setting `"PeerSharing": false` explicitly is still useful when operating a node as a relay that synchronizes only with other known relays, for example.

Currently, the `PeerSharing` key is removed from https://book.play.dev.cardano.org/environments/mainnet/config-legacy.json For reference, perhaps the key should continue to be included in the available `config-legacy.json` file.