Add `stellar network root-account` command to output root account master key and address

[leighmcculloch]

What problem does your feature solve?

Deriving the root account's master key and address for a network currently requires using stellar-core convert-id with some shell pipeline glue. For example, in quickstart:

NETWORK_ID=$(printf "$NETWORK_PASSPHRASE" | sha256sum | cut -f 1 -d " ")
network_id_keys=$(stellar-core convert-id $NETWORK_ID | awk -F': ' '/strKey: /{print $2}' | tail -2)
NETWORK_ROOT_SECRET_KEY=$(printf "$network_id_keys" | head -1)
NETWORK_ROOT_ACCOUNT_ID=$(printf "$network_id_keys" | tail -1)

This requires stellar-core to be available and involves brittle parsing of its output. The same logic would be useful in ops tooling for network setup.

What would you like to see?

Add a stellar network root command with subcommands to output the root account's master key and address for a given network. For example:

stellar network root-account master-key --network testnet
# outputs: S...

stellar network root-account address --network testnet
# outputs: G...

The command would derive these from the network passphrase, the same way stellar-core convert-id does today, but without needing stellar-core installed.

What alternatives are there?

• Continue using stellar-core convert-id with shell parsing, as quickstart does today. This requires stellar-core to be available and is fragile.
• Implement the derivation in a standalone script using the JS SDK's Keypair.master(Networks.TESTNET) equivalent. This works but adds a dependency on the JS SDK instead of keeping it in the CLI.

[leighmcculloch]

@mootz12 @fnando Based off our decision for #2413 to add the network id to the network info JSON output, maybe we should add the network root account info to that JSON blob too?

In some ways it feels like it does cross-over into a specialised area... so maybe it doesn't belong there. But every network has a root account so seems fine.