Enable OpenSSL certificates

[eschnett]

I'm working on replacing MbedTLS by OpenSSL in Julia. See JuliaLang/julia#56708. This allows (requires?) enabling OpenSSL certificates.

[fxcoudert]

ping @StefanKarpinski on this? This is required for the julia PR at JuliaLang/julia#56708 given that MbedTLS is reaching end-of-support.

[giordano]

I'm not familiar with the codebase, but the suggested changes look reasonable to me and we've already got an approval. If anyone has further improvements they can be made in a follow up PR, but this is important to move JuliaLang/julia#56708 forward, so I'm going to merge it.

[StefanKarpinski]

Sorry for the slow reply. This seems reasonable if we can handle OpenSSL certificates now. The issue before was that we couldn't and Downloads would choke on cert files that contained no regular certs. The only potential improvements here is that we could detect whether we're using MbedTLS or OpenSSL and automatically skip if needed, but this is fine unless there are problems. I don't really think it makes much sense for the NetworkOptions version to be decoupled from the Julia version anyway. It's theoretically possible, but just not very useful imo.