Feature: CA-specific HTTP proxy settings

[mkauf]

A customer wants to use two different ACME CAs: one for internal domains (intranet), one for external domains (public websites). The ACME CA for the external domains is only reachable via an HTTP proxy. The HTTP proxy can only connect to the external ACME CA (not to the internal ACME CA).

Currently, it's not possible to configure mod_md for this scenario.

Possible solutions:

• Allow the MDHttpProxy setting to be set per MDomain
• Add a setting to skip the HTTP proxy for some specific hosts (curl option CURLOPT_NOPROXY)

I think that the first approach is better and easier to configure.

What do you think about this? I can submit a PR if you like one of the solutions.

[icing]

I also like the first approach more. Feel free to have a go at it.