Implement ChainObserver adapter configuration in aggregator

[jpraynaud]

Why

The stress test does not work anymore as PallasChainObserver is not compatible with the mock-cardano-cli. When starting the aggregator, the stress test panics with the following error:

Error: Dependencies Builder can not create aggregator runner

Caused by:
    Dependency initialization error: «cannot create aggregator runner: failed to retrieve current epoch.» with additional nested error: 'general error
    
    Caused by:
        0: PallasChainObserver Failed to create new client
        1: error connecting bearer
        2: Invalid argument (os error 22)'.

What

Make the aggregator use a ChainAdapter based on a configuration parameter and fix the stress test.

How

• Add a configuration parameter that allows to select the ChainObserver in the aggregator (as in EraAdapter)
• Make the aggregator of the stress test use CardanoChainCliObserver
• Make the aggregator of end to end test use PallasChainObserver
• Make the aggregator of the infrastructure use PallasChainObserver
• Update documentation