Requests: declarative and unified interface

[ksol]

This PR changes how methods are declared in order to be as consistent as possible in the way it deals with url params, query params, payload, and so on.

As a side-effect, a declarative interface was setup in order to both make dead simple to add an endpoint, and to ensure that all methods stay consistent. See each existing endpoint in order to see the new syntax for declaring endpoints.

Implementation notes

• paths follow the URI Template format
• templates variables are checked in the arguments received. If some are optionnal, they need to be declared as such
• the payload can be automatically wrapped in a root key
• using a non-authenticated connection is more explicit than previously

The specs had to be slightly updated in order to reflect the changes in the API, but they were modified as little as possible.

[aurelien-reeves-scalingo]

Looks nice!

Could be a good addition to identify update path and add it in an update guide in the readme, the changelog, or a dedicated document

[ksol]

It's intended :)