[VOL-3386] Add support for secure gRPC in openolt-agent

The init script of the openolt service may start with '--enable-tls <TLS_OPTION>' argument for the gRPC server.
Default is insecure with no '--enable-tls' argument.
The TLS capability depends upon the certificates stored at the keystore/ directory: 1. root.crt (CA public key), 2. server.crt (public key), 3.server.key (private key).
Four unit tests are added for the secure gRPC server which work with the keystore-test/ directory.
The certificates stored at the keystore-test/ directory are self-signed certificates, valid until Apr 11 23:16:58 2031 GMT.

Change-Id: I4d18a98a0193f501f922360c79f54b0fcedf14a5
diff --git a/agent/test/keystore-test/server.key b/agent/test/keystore-test/server.key
new file mode 100644
index 0000000..371ceeb
--- /dev/null
+++ b/agent/test/keystore-test/server.key
@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
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+pvZKgzwl2RaJTlr77Qz6lsa4
+-----END PRIVATE KEY-----