Speed up Lodestar keystore import

[nflaig]

Lodestar does not natively support importing keystores with different password in an efficient way right now (see https://github.com/ChainSafe/lodestar/issues/5249) that's why each keystore is imported individually right now via validator import command, but this is really slow, and redundant if keystores already exist after first startup.

We can speed this up significantly by directly writing keystores in Lodestar persistent keys backend, the outcome is exactly the same, we skip keys that are already imported, and put them into keystoresDir and secretsDir of Lodestar in the expected format.

This is the approach described in protolambda/eth2-val-tools and there is no downside in doing it like this.

It could further be considered if we wanna remove keystores from the persistent keys backend if those are no longer part of charon/validator_keys directory but this was not done previously as well and can be implemented separately if desired.