Set safe block hash to justified (#3347)

## Issue Addressed

Closes https://github.com/sigp/lighthouse/issues/3189.

## Proposed Changes

- Always supply the justified block hash as the `safe_block_hash` when calling `forkchoiceUpdated` on the execution engine.
- Refactor the `get_payload` routine to use the new `ForkchoiceUpdateParameters` struct rather than just the `finalized_block_hash`. I think this is a nice simplification and that the old way of computing the `finalized_block_hash` was unnecessary, but if anyone sees reason to keep that approach LMK.

testing/execution_engine_integration/Cargo.toml

@@ -20,3 +20,4 @@ ethers-providers = { git = "https://github.com/gakonst/ethers-rs", rev = "02ad93
 deposit_contract = { path = "../../common/deposit_contract" }
 reqwest = { version = "0.11.0", features = ["json"] }
 hex = "0.4.2"
+fork_choice = { path = "../../consensus/fork_choice" }