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>

validator_client/src/cli.rs

@@ -251,8 +251,9 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                 .takes_value(false),
         )
         .arg(
-            Arg::with_name("private-tx-proposals")
-                .long("private-tx-proposals")
+            Arg::with_name("builder-proposals")
+                .long("builder-proposals")
+                .alias("private-tx-proposals")
                 .help("If this flag is set, Lighthouse will query the Beacon Node for only block \
                     headers during proposals and will sign over headers. Useful for outsourcing \
                     execution payload construction during proposals.")
@@ -271,4 +272,22 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                      for payload construction, where a strict fee recipient check will still be applied.")
                 .takes_value(false),
         )
+        .arg(
+            Arg::with_name("builder-registration-timestamp-override")
+                .long("builder-registration-timestamp-override")
+                .alias("builder-registration-timestamp-override")
+                .help("This flag takes a unix timestamp value that will be used to override the \
+                    timestamp used in the builder api registration")
+                .takes_value(true),
+        )
+        .arg(
+            Arg::with_name("gas-limit")
+                .long("gas-limit")
+                .value_name("INTEGER")
+                .takes_value(true)
+                .help("The gas limit to be used in all builder proposals for all validators managed \
+                    by this validator client. Note this will not necessarily be used if the gas limit \
+                    set here moves too far from the previous block's gas limit. [default: 30,000,000]")
+                .requires("builder-proposals"),
+    )
 }