ci: add check rust formating github workflow

Signed-off-by: Ruowen Qin <ruowenq2@illinois.edu>

Author: chinrw

.github/workflows/rustfmt.yml

@@ -0,0 +1,35 @@
+name: Formatting check
+
+on:
+  push:
+    branches: [main, ci]
+    paths-ignore:
+      - docs/**
+      - tools/**
+      - README.md
+  pull_request:
+    branches: [main]
+    paths-ignore:
+      - docs/**
+      - tools/**
+      - README.md
+
+jobs:
+  formatting:
+    if: github.repository == 'rex-rs/rex'
+    name: cargo fmt
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: switch rust nightly
+        run: |
+          rustup default nightly
+          rustup component add rustfmt
+      - name: formatting rex code
+        run: cargo fmt
+      - name: formatting samples code
+        run: |
+          for d in $(find ./samples -name Cargo.toml); do
+            echo "→ Processing $d"
+            cargo fmt --manifest-path $d --verbose --check
+          done