Improve the application and introduce paf-le as a standalone package

[dinosaure]

This PR has a minor commit about the application and introduce what we did into paf-le. The main difference is the usage of http-mirage-client as the HTTP client to do HTTP requests to Let's encrypt instead of an handmade hidden module which is an old version of http-mirage-client. The goal is to delete duplication of code and finally use happy-eyeballs as the DNS resolver instead of requiring a gethostbyname.

/cc @hannesm

[hannesm]

This looks mostly good. But I hesitate a bit about the module name LE_server -- which is actually the HTTP server for the LE http challenge. My hesitation is that with LE_server I immediately think about the server side of let's encrypt (while this is the HTTP server side for the LE client side).

Would LE_http_server or LE_http be a fine module name?

[hannesm]

Thanks @dinosaure, only some minor comments. Good to be merged.

To avoid confusion:

• there is a let's encrypt ALPN challenge not used here (but supported in the library, requires work on configuration (server): allow certificate selection to be dependent on the ALPN mirleft/ocaml-tls#465 (where a special self-signed certificate is provided when LE asked for a specific ALPN name))
• this new package uses the let's encrypt HTTP challenge (where a .well-known/... is used as response to the challenge)

ALPN is still used in here for HTTP 1.1 and HTTP 2 support :)

[hannesm]

Thanks, I'll merge this now, but wait for a release until the big split [tm] has settled.