proxy: refactor request/response format

[jkasarherou]

Actually, the proxy uses a mix of headers, query parameters, and field JSON encoded in the request body. We should stop using headers at all (we have full control on the request/response body format) and  use JSON.
Example: To set properties on a container creation:
instead of sending parameters as headers (which is confusing for parameters case sensitivity), we should simply use the JSON encoded body.

[jfsmig]

🆙

[jkasarherou]

let's do this

[jfsmig]

A lot of improvements have been done here, in a round ended with #686 .