Simplify WORKSPACE.bzlmod and add CI test for Bzlmod

- All workspace suffix are removed from WORKSPACE.bzlmod, since we get
  all dependencies and toolchains from Bzlmod.
- Added build:bzlmod bazelrc section, we have to override two flags to
  make sure toolchains are picked up correctly.
- Added zstd-jni as dependency
- Added postsubmit_bzlmod.yml for testing the Bzlmod build.

Tested at https://buildkite.com/bazel/bazel-bazel-with-bzlmod/builds/3

Closes #14312.

PiperOrigin-RevId: 411855954
diff --git a/.bazelrc b/.bazelrc
index 0fb596e..cb59636 100644
--- a/.bazelrc
+++ b/.bazelrc
@@ -29,5 +29,11 @@
 
 build:macos --macos_minimum_os=10.10
 
+# Enable Bzlmod
+build:bzlmod --experimental_enable_bzlmod
+# TODO(pcloudy): The following should be removed after fixing https://github.com/bazelbuild/bazel/issues/14279
+build:bzlmod --crosstool_top=@rules_cc.0.0.1.cc_configure.local_config_cc//:toolchain
+build:bzlmod --xcode_version_config=@rules_cc.0.0.1.cc_configure.local_config_xcode//:host_xcodes
+
 # User-specific .bazelrc
 try-import user.bazelrc