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.

Cargo.lock

@@ -1932,6 +1932,7 @@ dependencies = [
  "ethers-providers",
  "execution_layer",
  "exit-future",
+ "fork_choice",
  "futures",
  "hex",
  "reqwest",
@@ -1958,6 +1959,7 @@ dependencies = [
  "eth2_ssz_types",
  "ethers-core",
  "exit-future",
+ "fork_choice",
  "futures",
  "hex",
  "jsonwebtoken",