Bazel: introduce buildifier formatting

This introduces tooling and enforcement for formatting bazel files.

The tooling is provided as a bazel run target from
[keith/buildifier-prebuilt](https://github.com/keith/buildifier-prebuilt).

This is used in a [`pre-commit`](https://pre-commit.com/) hook for those
having that installed. In turn this is used in a CI check. Relying on a
`pre-commit` action gives us easy checking that buildifying did not
change anything in the files and printing the diff, without having to
hand-roll the check ourselves.

This enforcement will make usage of gazelle easier, as gazelle itself
might reformat files, even outside of `go`. Having them properly
formatted will allow gazelle to leave them unchanged, without needing
to configure awkward exclude directives.

.github/workflows/buildifier.yml

@@ -0,0 +1,26 @@
+name: Check bazel formatting
+
+on:
+  pull_request:
+    paths:
+      - "**.bazel"
+      - "**.bzl"
+    branches:
+      - main
+      - "rc/*"
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - uses: pre-commit/action@646c83fcd040023954eafda54b4db0192ce70507
+        with:
+          extra_args: buildifier --all-files --show-diff-on-failure
+      - if: failure()
+        run: |
+          echo "In order to format all files, please run:"
+          echo "  bazel run //:buildifier"