Append custom request extension (get/set only)

[Voronar]

The feature is based on work are done here which introduced registered extensions map with custom extension messages and response extension JSON-formatting.

Now it is possible to append custom extension to set/get request in JSON format:

gnmic -a 127.0.0.1 --insecure \
	--proto-dir protos \
	--proto-file ext1.proto \
	--registered-extensions 10:package1.Message1 \
	--request-extensions '{ "10": { "msg1_field": 10 } }' \
	set --target node1 --path-replace root:/ --replace-value '{}'

If the response contains the same extension it would be automatically JSON-formatted based on feature I mentioned above.

[karimra]

Thanks for the contribution and for making the changes.