Create keystore files for new validators

[xenowits]

🎯 Problem to be solved

The charon alpha add-validators-alpha command doesn't create keystore files (keystore-i.json & keystore-i.txt) for newly created validators.

🛠️ Proposed solution

Create keystore files for each new validator and store them in each node's individual directory:

• node0/validator_keys
• node1/validator_keys
• ....
• nodeN/validator_keys

Keystore files should be named starting with keystore-<numVals>.json. For example, if a cluster already has 5 validator keys in its validator_keys/ directory and we try adding 2 more, the new keystore files should be:

• keystore-5.json
• keystore-5.txt
• keystore-6.json
• keystore-6.txt

🧪 Tests

• Tested by new automated unit/integration/smoke tests
• Manually tested on core team/canary/test clusters
• Manually tested on local compose simnet