Consume Opendatasoft Explore API

Summary:
The data.economie.gouv.fr API uses Opendatasoft Explore API,
a commercial product with an API to query datasets information.

Plan is to query attachments for FANTOIR dataset, to know when
a new file is published.

This library implements all the methods, but doesn't currently
support API optional arguments.

Ref T1750

Test Plan: Used in D2738.

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1750

Differential Revision: https://devcentral.nasqueron.org/D2745