Some fixes for AWS/S3

[ochaton]

I discovered that operations without empty description of operation.input does not work at all. I've added if into aws/init.lua and aws/request/build.lua

• validate operation.input first before validate against it. s3/listBuckets does not accept any input.validator

s3_patch (aws/init.lua) seems to be not quite well written. When we change request.host we should change request.headers.Host as well, because of the signature. Also, operations such as listObjectsV2 and other meta operations requires non-empty path.

• refix S3 fixes path: if path=="" we need to restore path="/"

The last part is not quite good :( It would be nice if we could parse xml output into Lua tables, we have operation.output descriptions for it. To do that we need to distinguish streaming content and API responses.

Streaming content (GetObject mostly, maybe GetObjectTorrent) it would be better to not read entire body into Lua memory, but provide body_reader from resty.http instead. Although I'm not sure that this behaviour should be fixed the way I did it

• return raw body reader if operation output expects blob or streaming content

P.S. raw-api contains 17 "streaming":true lines, but necessarily all of the Outputs

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