feat: bootstrap bun toolchain skeleton and smoke test

Co-authored-by: Eriyc <50216491+Eriyc@users.noreply.github.com>

bun/toolchain.bzl

@@ -0,0 +1,21 @@
+BunToolchainInfo = provider(fields = ["bun_bin", "version"])
+
+
+def _bun_toolchain_impl(ctx):
+    return [
+        platform_common.ToolchainInfo(
+            bun = BunToolchainInfo(
+                bun_bin = ctx.executable.bun,
+                version = ctx.attr.version,
+            ),
+        ),
+    ]
+
+
+bun_toolchain = rule(
+    implementation = _bun_toolchain_impl,
+    attrs = {
+        "bun": attr.label(allow_single_file = True, executable = True, cfg = "exec"),
+        "version": attr.string(mandatory = True),
+    },
+)