Args array erroneously set to ScriptArgs in /v1/agent APIs

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This captures an issue from from https://groups.google.com/d/msgid/consul-tool/9dca61e6-26b0-4315-a891-2713756f768a%40googlegroups.com?utm_medium=email&utm_source=footer.

It looks like we didn't JSON-annotate fields correctly, so Args ended up as ScriptArgs in the /v1/agent check and service registration APIs. We also left the Args field out of the API client. We need to add this to the API client and fix up the documentation so it works with what we shipped.

We should rename the field Args so that the config file syntax matches the JSON in the API, but we will need to retain backwards compatibility with ScriptArgs since that has already shipped.

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This is another issue related to #3179.