Update attestation rewards API for Electra (#6819)

Closes:

- https://github.com/sigp/lighthouse/issues/6818


  Use `MAX_EFFECTIVE_BALANCE_ELECTRA` (2048) for attestation reward calculations involving Electra.

Add a new `InteropGenesisBuilder` that tries to provide a more flexible way to build genesis states. Unfortunately due to lifetime jank, it is quite unergonomic at present. We may want to refactor this builder in future to make it easier to use.

beacon_node/genesis/src/lib.rs

@@ -7,6 +7,6 @@ pub use eth1::Eth1Endpoint;
 pub use eth1_genesis_service::{Eth1GenesisService, Statistics};
 pub use interop::{
     bls_withdrawal_credentials, interop_genesis_state, interop_genesis_state_with_eth1,
-    interop_genesis_state_with_withdrawal_credentials, DEFAULT_ETH1_BLOCK_HASH,
+    InteropGenesisBuilder, DEFAULT_ETH1_BLOCK_HASH,
 };
 pub use types::test_utils::generate_deterministic_keypairs;