node, sync: use new Execution API client/server

[canepat]

This PR changes the node and sync modules to use the client/server abstractions built around the execution interface. Specifically:

• create Execution API DirectClient and Server (disabled by config for now) in node module facade
• migrate usage of Execution API Client in sync module
• bridge Execution API DirectClient from node to sync facade in silkworm main

Extras

• extract sample block generation as common fixture in unit tests
• improve readability by adding some type alias
• handle some more error conditions in PoS sync

This PR depends on #2004