Etherlink: Add multichain feature flag

What

This MR introduces a new feature flag multichain.

Why

Currently tezos only supports a single L2 solution: etherlink. Our goal is to gradually introduce support for multiple chains while keeping the production code unaffected. This approach allows us to merge changes into master quickly and safely to implement upcoming features.

How

This flag closely follows the existing dal flag.

Manually testing the MR

With multichain disabled, the existing behaviour remains unchanged.

The first feature added behind the flag is chain_configurations the structure responsible for configuring future chains. New tests were added showing that when the flag is activated, the chain_id can be manipulated and correctly processed in this use case.