ci: add cargo hack feature powerset test

This commit updates CI to use `cargo hack` to test the feature powerset,
ensuring that we can catch feature interplay related breakages early.

Unlike w/ the main Rustls repo the webpki powerset is small and the test
completes in <30s, so it's fair to include in the default CI instead of
needing to be separated into a separate daily tests workflow.

.github/workflows/ci.yml

@@ -292,3 +292,21 @@ jobs:
 
       - name: check no-std mode
         run: cargo check --target $NOSTD_TARGET ${{ matrix.features  }}
+
+  feature-powerset:
+    name: Feature Powerset
+    runs-on: ubuntu-20.04
+    steps:
+      - name: Checkout sources
+        uses: actions/checkout@v4
+        with:
+          persist-credentials: false
+
+      - name: Install stable toolchain
+        uses: dtolnay/rust-toolchain@stable
+
+      - name: Install cargo hack
+        uses: taiki-e/install-action@cargo-hack
+
+      - name: Check feature powerset
+        run: cargo hack check --feature-powerset --no-dev-deps