Add minimal example of server with encryption

Also add example certificate and private key

examples/example-private-key.pem

+-----BEGIN PRIVATE KEY-----
+MIIEvwIBADANBgkqhkiG9w0BAQEFAASCBKkwggSlAgEAAoIBAQCfaDB7pK/fmP/I
+7IusSK8lTCBnPZghqIbVLt2QHYAMoEF1CaF4F4rxo2vl1Mt8gwsq4T3osQFZMvnL
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+sy51vAyEtdBC7jgAU96ZVeQ06nryDq1D2EpoVMElqNyL46Jo3lnKbGquGKzXzQYU
+BN32/scDAgMBAAECggEBAJE/mo3PLgILo2YtQ8ekIxNVHmF0Gl7w9IrjvTdH6hmX
+HI3MTLjkmtI7GmG9V/0IWvCjdInGX3grnrjWGRQZ04QKIQgPQLFuBGyJjEsJm7nx
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+osZOhe09WuMCgYEA0G9aFCnwjUqIFjQiDFP7gi8BLqTFs4uE3Wvs4W11whV42i+B
+ms90nxuTjchFT3jMDOT1+mOO0wdudLRr3xEI8SIF/u6ydGaJG+j21huEXehtxIJE
+aDdNFcfbDbqo+3y1ATK7MMBPMvSrsoY0hdJq127WqasNgr3sO1DIuima3SECgYEA
+nkM5TyhekzlbIOHD1UsDu/D7+2DkzPE/+oePfyXBMl0unb3VqhvVbmuBO6gJiSx/
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+srJnjF0H8oKmAY6hw+1Tm/n/b08p+RyL48TgVSE2vhUCgYA3BWpkD4PlCcn/FZsq
+OrLFyFXI6jIaxskFtsRW1IxxIlAdZmxfB26P/2gx6VjLdxJI/RRPkJyEN2dP7CbR
+BDjb565dy1O9D6+UrY70Iuwjz+OcALRBBGTaiF2pLn6IhSzNI2sy/tXX8q8dBlg9
+OFCrqT/emes3KytTPfa5NZtYeQ==
+-----END PRIVATE KEY-----

examples/minimal-server-with-encryption.py

+import sys
+sys.path.insert(0, "..")
+import time
+
+
+from opcua import ua, Server
+
+
+if __name__ == "__main__":
+
+    # setup our server
+    server = Server()
+    server.set_endpoint("opc.tcp://0.0.0.0:4841/freeopcua/server/")
+
+    # load server certificate and private key. This enables endpoints
+    # with signing and encryption.
+    server.load_certificate("example-certificate.der")
+    server.load_private_key("example-private-key.pem")
+
+    # setup our own namespace, not really necessary but should as spec
+    uri = "http://examples.freeopcua.github.io"
+    idx = server.register_namespace(uri)
+
+    # get Objects node, this is where we should put our custom stuff
+    objects = server.get_objects_node()
+
+    # populating our address space
+    myobj = objects.add_object(idx, "MyObject")
+    myvar = myobj.add_variable(idx, "MyVariable", 6.7)
+    myvar.set_writable()    # Set MyVariable to be writable by clients
+
+    # starting!
+    server.start()
+    try:
+        count = 0
+        while True:
+            time.sleep(1)
+            count += 0.1
+            myvar.set_value(count)
+    finally:
+        #close connection, remove subcsriptions, etc
+        server.stop()