feat(transport): Add support client mTLS (#77)

This commit adds a simple API for specifying the TLS certificate a GRPC
client will present (via the same `Identity` wrapper as a server cert is
configured). It also adds an API to specify which CA certificate client
TLS certificates will be validated against for servers.

tonic-examples/data/tls/client1.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
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+-----END PRIVATE KEY-----

tonic-examples/data/tls/client1.pem

@@ -0,0 +1,19 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----

tonic-examples/data/tls/client2.key

@@ -0,0 +1,28 @@
+-----BEGIN PRIVATE KEY-----
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+-----END PRIVATE KEY-----

tonic-examples/data/tls/client2.pem

@@ -0,0 +1,19 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----

tonic-examples/data/tls/client_ca.pem

@@ -0,0 +1,19 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----