feat: generate keypairs in a batch

[gluckzhang]

For each node, the validator_service role starts up Firedancer with an unstaked identity keypair without voting by default, unless users override the config and manually place the keypairs.

It would be nice to automate key generations as these are repeated tasks for every node. The role should create a folder for each host, and generate the following three keys in the control node:

• validator-keypair.json
• authorized-withdrawer-keypair.json
• vote-account-keypair.json

Then, only place validator-keypair.json and vote-account-keypair.json in the target host, under path /home/{{ service_user }}. At last, output a debug message that reminds the operator to backup the keypairs, and the recovery phrases.