Fix chain_id value in config/deposit_contract RPC method (#2659)

## Issue Addressed

This PR addresses issue #2657

## Proposed Changes

Changes `/eth/v1/config/deposit_contract` endpoint to return the chain ID from the loaded chain spec instead of eth1::DEFAULT_NETWORK_ID which is the Goerli chain ID of 5.

Co-authored-by: Michael Sproul <michael@sigmaprime.io>

beacon_node/http_api/tests/main.rs

@@ -3,4 +3,5 @@
 
 pub mod common;
 pub mod fork_tests;
+pub mod interactive_tests;
 pub mod tests;