Import grpcio 0.6.0

And add metadata files using the following command:
get_rust_pkg.py --add3prf -v grpcio-0.6.0 -o grpcio

Test: none
Change-Id: I53cc0feb5c9d24eacb62331b968cab4ec85f60a6
diff --git a/src/security/mod.rs b/src/security/mod.rs
new file mode 100644
index 0000000..f2c2bad
--- /dev/null
+++ b/src/security/mod.rs
@@ -0,0 +1,10 @@
+// Copyright 2020 TiKV Project Authors. Licensed under Apache-2.0.
+
+mod credentials;
+
+pub use self::credentials::{
+    CertificateRequestType, ChannelCredentials, ChannelCredentialsBuilder, ServerCredentials,
+    ServerCredentialsBuilder, ServerCredentialsFetcher,
+};
+
+pub(crate) use self::credentials::server_cert_fetcher_wrapper;