Problem: `priv_validator_state.json` may get corrupted

[tomtau]

@borischeuk-crypto came across an issue where priv_validator_state.json was empty and the nitro helper would crash with:

failed to get a state syncing helper: Error(Context { kind: SyncError, source: Some(Message("error parsing .../.tmkms/state/priv_validator_state.json: EOF while parsing a value at line 1 column 0")), backtrace: Some(   0: tmkms_nitro_helper::command::start

There are probably a few ways how this can be improved:

1. check the state syncing helper code whether atomic writing via tempfile can be improved (also not sure if the helper @borischeuk-crypto used had https://github.com/crypto-com/tmkms-light/pull/115/files )
2. improve the error message to e.g. give some troubleshooting suggestion (re-initialize the file with the latest block height?)
3. add a different option for storing/retrieving priv_validator_state.json... e.g. with DynamoDB instead of a local file

[tomtau]

For reference, a code snippet for that potential DynamoDB fallback option

...
client.update_item().table_name(table_name).key("validator-id", AttributeValue::S(validator_id))
    .condition_expression("Height < :he OR (Height = :he AND Round < :ro) OR (Height = :he AND Round = :ro AND Step < :st)")
    .update_expression("SET BlockId = :bl, Height = :he, Round = :ro, Step = :st")
    .expression_attribute_values(":bl", AttributeValue::S(state.block_id.unwrap_or_default().to_string()))
    .expression_attribute_values(":he", AttributeValue::N(state.height.to_string()))
    .expression_attribute_values(":ro", AttributeValue::N(state.round.to_string()))
    .expression_attribute_values(":st", AttributeValue::N(state.step.to_string()))
    .send().await;