Implements a fully-featured acme integration

[FlorentinDUBOIS]

Hello 👋 ,

Currenly, Sozu provides a command line with a sub command that allow to order let's encrypt certificate. According to the pull request #925, this is not the experience that we want.

We would like to provide a full-featured acme integration that plug with Sozu and handle the lifecycle of certificates including ordering, loading into Sozu and renewals. This experience will be really close to the one that caddy provides.

[FlorentinDUBOIS]

After some thoughts, I would like to propose something that could run close to the Sozu instance or not on the same server. I will try to explicit myself in the section below.

We have started the work to change the internal and external communication to protobuf. This effort has been achieved to ease the integration of new tools around Sozu (you could take a look to dedicated milestones v1.0.0 and v1.1.0) and provide some tooling and guarantee on the message format for future evolution.

What I mean is that we could create an API that provides us a way to interact remotely with Sozu. Besides, it will help to achieved the following issue #149

[FlorentinDUBOIS]

Besides, we should not forget this issue #686 as well