Lint against panicky calls in async functions (#3250)

## Description

Add a new lint to CI that attempts to detect calls to functions like `block_on` from async execution contexts. This lint was written from scratch exactly for this purpose, on my fork of Clippy: https://github.com/michaelsproul/rust-clippy/tree/disallow-from-async

## Additional Info

- I've successfully detected the previous two issues we had with `block_on` by running the linter on the commits prior to each of these PRs: https://github.com/sigp/lighthouse/pull/3165, https://github.com/sigp/lighthouse/pull/3199.
- The lint runs on CI with `continue-on-error: true` so that if it fails spuriously it won't block CI.
- I think it would be good to merge this PR before https://github.com/sigp/lighthouse/pull/3244 so that we can lint the extensive executor-related changes in that PR.
- I aim to upstream the lint to Clippy, at which point building a custom version of Clippy from my fork will no longer be necessary. I imagine this will take several weeks or months though, because the code is currently a bit hacky and will need some renovations to pass review.

.github/custom/clippy.toml

@@ -0,0 +1,21 @@
+disallowed-from-async-methods = [
+    "tokio::runtime::Handle::block_on",
+    "tokio::runtime::Runtime::block_on",
+    "tokio::task::LocalSet::block_on",
+    "tokio::sync::Mutex::blocking_lock",
+    "tokio::sync::RwLock::blocking_read",
+    "tokio::sync::mpsc::Receiver::blocking_recv",
+    "tokio::sync::mpsc::UnboundedReceiver::blocking_recv",
+    "tokio::sync::oneshot::Receiver::blocking_recv",
+    "tokio::sync::mpsc::Sender::blocking_send",
+    "tokio::sync::RwLock::blocking_write",
+]
+async-wrapper-methods = [
+    "tokio::runtime::Handle::spawn_blocking",
+    "task_executor::TaskExecutor::spawn_blocking",
+    "task_executor::TaskExecutor::spawn_blocking_handle",
+    "warp_utils::task::blocking_task",
+    "warp_utils::task::blocking_json_task",
+    "validator_client::http_api::blocking_signed_json_task",
+    "execution_layer::test_utils::MockServer::new",
+]

.github/workflows/test-suite.yml

@@ -252,6 +252,23 @@ jobs:
       run: make lint
     - name: Certify Cargo.lock freshness
       run: git diff --exit-code Cargo.lock
+  disallowed-from-async-lint:
+    name: disallowed-from-async-lint
+    runs-on: ubuntu-latest
+    needs: cargo-fmt
+    continue-on-error: true
+    steps:
+    - uses: actions/checkout@v1
+    - name: Install SigP Clippy fork
+      run: |
+        cd ..
+        git clone https://github.com/michaelsproul/rust-clippy.git
+        cd rust-clippy
+        git checkout 31a49666ccfcd7963b63345d6ce757c373f22c2a
+        cargo build --release --bin cargo-clippy --bin clippy-driver
+        cargo build --release --bin cargo-clippy --bin clippy-driver -Zunstable-options --out-dir $(rustc --print=sysroot)/bin
+    - name: Run Clippy with the disallowed-from-async lint
+      run: make nightly-lint
   check-msrv:
     name: check-msrv
     runs-on: ubuntu-latest

.gitignore

@@ -8,3 +8,4 @@ perf.data*
 *.tar.gz
 /bin
 genesis.ssz
+/clippy.toml

Makefile

@@ -12,6 +12,7 @@ AARCH64_TAG = "aarch64-unknown-linux-gnu"
 BUILD_PATH_AARCH64 = "target/$(AARCH64_TAG)/release"
 
 PINNED_NIGHTLY ?= nightly
+CLIPPY_PINNED_NIGHTLY=nightly-2022-05-19
 
 # List of all hard forks. This list is used to set env variables for several tests so that
 # they run for different forks.
@@ -145,6 +146,14 @@ lint:
 		-A clippy::upper-case-acronyms \
 		-A clippy::vec-init-then-push
 
+# FIXME: fails if --release is added due to broken HTTP API tests
+nightly-lint:
+	cp .github/custom/clippy.toml .
+	cargo +$(CLIPPY_PINNED_NIGHTLY) clippy --workspace --tests -- \
+		-A clippy::all \
+		-D clippy::disallowed_from_async
+	rm clippy.toml
+
 # Runs the makefile in the `ef_tests` repo.
 #
 # May download and extract an archive of test vectors from the ethereum