Add support for x-ms-paths, x-ms-odata, and x-ms-error-response

[rylev]

There are two issues with this PR:

• The tests fail because the newest json we're testing against (apimapis.json) includes empty "definitions" and "parameters" keys while the other json files do not include these when they are empty. In other words, sometimes they are included when empty, and sometimes they are not. We should think about patching assert_json_diff to allow for empty objects to be treated as though the key is not present.
• The x_ms_odata key is modeled as an Option<String>. The String in this case is always a reference to another file with the object's definition. Unfortunately, normally references are json objects with one key "$ref", but in this case it's just the string value without the key. We should think about how to unify these difference types of references.

This should close #3 though there is still one extension "x-ms-text" which doesn't seem to be in any of the Azure REST spec json files.