Convert to `cargo_embargo`

Bug: 302021139
Test: m libmls_rs_uniffi in local checkout
Change-Id: I4a6c562bf09b844b71b91e71ca830d38bf3e71c0
diff --git a/mls/mls-rs-crypto-boringssl/cargo_embargo.json b/mls/mls-rs-crypto-boringssl/cargo_embargo.json
new file mode 100644
index 0000000..de66107
--- /dev/null
+++ b/mls/mls-rs-crypto-boringssl/cargo_embargo.json
@@ -0,0 +1,4 @@
+{
+    "extra_cfg": ["mls_build_async"],
+    "run_cargo": false
+}