feat: Validium DA Manager example

[ilitteri]

DISCLAIMER: This PR makes use of #1016 changes, if you want to run the example please follow the instructions of our internal PR.

DA Manager (separate binary)

A DA Manager is introduced, a component that periodically queries for new batches and uploads their pubdata to some data availability solution, as a separate binary. The way this works is the following:

• To construct the pubdata, the operator will query Postgres for the four components of pubdata, then convert it into its byte representation using the logic in the
  pub fn construct_pubdata(&self) -> Vec<u8> method of the L1BatchWithMetadata struct.
• A user wanting to incorporate a new DA solution simply writes their own DA manager in the programming language they prefer, as an application that uses the endpoint introduced above to periodically query for new pubdata and post it to the DA server. For this scenario, we should provide some examples showing what you should implement for your own DA solution.
• This DA Manager runs independently, from the operator, i.e. as an entirely separate OS process or even on a different machine (in particular, it’s independent from both the eth_tx_aggregator and the eth_tx_manager)