feat(admin) add multipart/form-data support to admin api (80% solution)

[bungle]

Summary

This adds multipart/form-data support to Admin API. But this is not a 100% solution as there turns our to be edge cases because of Lapis Framework that are not very easy to solve without rather complex changes.

How Lapis Reads the multipart/form-data:

1. On OpenResty Lapis uses lua-resty-upload to stream read multipart/form-data upload.
2. lua-resty-upload consumes the request body without writing it back, which means that you cannot read request body in our arguments parser.
3. That means that only place we can access multipart/form-data params in admin api is through Lapis self.req.params_post.
4. Lapis in turn does parsing of multipart/form-data to a key/value table. And because of that you cannot send same name in multipart/form-data twice, e.g. paths=/first and paths=/second OR paths[]=/first and paths[]=/second. But you can work-around that on client side by sending: paths[1]=/first and paths[2]=/second. That's why I call this 80% solution.

Should we wait for 100% solution? In my opinion not. This 80% solution is already more than enough to solve issues such as:
#3738
#3610

We can re-visit this later with 100% solution.

[kikito]

Looks good to me, but I think this needs one or two tests before we can merge it.

[bungle]

@kikito I added multipart/form-data to many it_content_types tests, but because issues in client, and because of this 80% solution, I needed to skip it in some tests. But in general this adds a lot of tests there.

resolved