[node] add option to generate node keys

[BulatSaif]

The chart already has the persistGeneratedNodeKey option to generate node keys. These keys are random and stored on disk. They are not known before deployment, so they cannot be used for testnet bootnode deployments.

Changes:

1. Add an option to generate node keys within helm secrets.
2. Replace --node-key $(cat /path/to/key) with --node-key-file /path/to/key. This is more secure, as the key is not exposed in the ps output. cat was previously used to fix the newline issue at the end of the file. Keys stored in Helm secrets do not have a trailing newline.