Transition Mithril Era Reader adapter to CardanoChain in devnet

[jpraynaud]

Why

We want to include Mithril era markers stored on the Cardano chain in the end to end test. This will help thoroughly test this mechanism. Moreover, the issue #1315 is replacing the CardanoCliChainObserver and we want to test the new PallasChainObserver before merging the PR #1403.

What

Make the signer and aggregator nodes use read the era markers from the Cardano chain (instead of relying on the dummy adapter) in the end to end test.

How

• Upgrade era generate-tx-datum command of the aggregator to export datum to a file
• Upgrade the devnet with the generation of a script that writes era markers to the Cardano chain
• Upgrade the end to end test so that signer and aggregator nodes use the CardanoChain era reader adapter