Adds gNMI extensions output formatting

[Voronar]

Resolves #761.

Input:

gnmic -a 127.0.0.1 --insecure \
	--proto-dir protos \
	--proto-file ext1.proto \
	--proto-file ext2.proto \
	--registered-extensions 10:package1.Message1 \
	--registered-extensions 20:package2.Message2 \
	get --target node1 --path root:/

Output:

{
	// ...
	"extension": [
    {
      "Ext": {
        "RegisteredExt": {
          "id": 10,
          "msg": "protobytes"
        }
      }
    },
    {
      "Ext": {
        "RegisteredExt": {
          "id": 20,
          "msg": "protobytes"
        }
      }
    }
  ],
	"decodedExtensions": {
		"10": { "ext1_message": { "msg1_field": 10 } },
		"20": { "ext2_message": { "msg2_field": 20 } }
	}
}

It should work for all types of responses (get, set, sub).

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[karimra]

Thanks for the contribution!
the --proto-dir and --proto-file were not added for this purpose but I guess it's a good way to reuse flags.