Builder Specs v0.2.0 (#3134)

## Issue Addressed

https://github.com/sigp/lighthouse/issues/3091

Extends https://github.com/sigp/lighthouse/pull/3062, adding pre-bellatrix block support on blinded endpoints and allowing the normal proposal flow (local payload construction) on blinded endpoints. This resulted in better fallback logic because the VC will not have to switch endpoints on failure in the BN <> Builder API, the BN can just fallback immediately and without repeating block processing that it shouldn't need to. We can also keep VC fallback from the VC<>BN API's blinded endpoint to full endpoint.

## Proposed Changes

- Pre-bellatrix blocks on blinded endpoints
- Add a new `PayloadCache` to the execution layer
- Better fallback-from-builder logic

## Todos

- [x] Remove VC transition logic
- [x] Add logic to only enable builder flow after Merge transition finalization
- [x] Tests
- [x] Fix metrics
- [x] Rustdocs


Co-authored-by: Mac L <mjladson@pm.me>
Co-authored-by: realbigsean <sean@sigmaprime.io>

beacon_node/execution_layer/src/test_utils/mod.rs

@@ -22,6 +22,7 @@ use types::{EthSpec, ExecutionBlockHash, Uint256};
 use warp::{http::StatusCode, Filter, Rejection};
 
 pub use execution_block_generator::{generate_pow_block, Block, ExecutionBlockGenerator};
+pub use mock_builder::{Context as MockBuilderContext, MockBuilder, Operation, TestingBuilder};
 pub use mock_execution_layer::MockExecutionLayer;
 
 pub const DEFAULT_TERMINAL_DIFFICULTY: u64 = 6400;
@@ -30,6 +31,7 @@ pub const DEFAULT_JWT_SECRET: [u8; 32] = [42; 32];
 
 mod execution_block_generator;
 mod handle_rpc;
+mod mock_builder;
 mod mock_execution_layer;
 
 /// Configuration for the MockExecutionLayer.