decouple eth2 from store and lighthouse_network

[dknopik]

Issue Addressed

• Modularize or Simplify the eth2 crate #6452 (partially)

Proposed Changes

Remove dependencies on store and lighthouse_network from eth2. This was achieved as follows:

• depend on enr and multiaddr directly instead of using lighthouse_network's reexports.
• make lighthouse_network responsible for converting between API and internal types.
• in two cases, remove complex internal types and use the generic serde_json::Value instead - this is not ideal, but should be fine for now, as this affects two internal non-spec endpoints which are meant for debugging, unstable, and subject to change without notice anyway. Inspired by Remove lighthouse_network dependency from eth2 crate #6679. The alternative is to move all relevant types to eth2 or types instead - what do you think?

Additional Info

In my opinion, this only partially fixes the issue above, as we probably still should split out lighthouse-specific features.