make l1_batch_commit_data_generator_mode a non-mandatory attribute

[mationorato]

discussion: matter-labs/zksync-era@main...lambdaclass:zksync-era:feat_validium_pubdata_abstraction#diff-1f5da47e2620f95f5ea03f9b1c51c9d7c8a2cfe53c09f2710c7377389dea185dR127

l1_batch_commit_data_generator_mode field in StateKeeperConfig is not marked as Option and has no serde(default) attribute which means that it’s always expected to be set. This means that if this change would be deployed to any existing env without updating configs beforehand, the node will enter the crash loop, because the required configuration option is not present.

Suggestion: Evaluate what makes more sense (Option or serde(default)) and implement it in a backward-compatible way.