httpcaddyfile: Add preferred_chains global option and issuer subdirective

[Klooven]

This PR adds support for the preferred_chains option in the Caddyfile; both as an option for the ACME issuer and as a global option. If merged, this PR would resolve #4185.

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The structure is the following:

preferred_chains [smallest] {
  root_common_name <common names...>
  any_common_name  <common names...>
}

The Caddyfile adapt tests added in this branch show how the options can be used both "locally" and globally. More details can be found in the issue linked above.

And, I'm of course happy to answer any questions that might arise 🙂

[CLAassistant]

All committers have signed the CLA.

[mholt]

This looks pretty good. The only nit I have is that the parsing code is basically identical between that of the issuer config and the global option.

Can you consolidate that?

Since package httpcaddyfile imports package caddytls, maybe the parsing code for this could be in caddytls as its own function.

[Klooven]

This looks pretty good. The only nit I have is that the parsing code is basically identical between that of the issuer config and the global option.

Can you consolidate that?

You're right. I'll have a look at it and send in an update to the PR when I've changed that.

[Klooven]

Ok, I managed to consolidate most of the stuff into a ParseCaddyfilePreferredChainsOptions function I created. Don't know if it's in the best place (I can move it if you feel like it should be in another place).

I encountered some issues on the way, so I don't know if the code is as elegant as it could be (comments welcome on this too). Did some testing and everything seemed to still work like before the "consolidation".

[mholt]

Great, looks good now! Thank you very much! Hope you'll contribute again.

[Klooven]

Thanks! This has been a great way to learn more about how Caddy works and to code with Go (which has been on my to-do list for too long 😄).