caddyhttp: Add MatchWithError to replace SetVar hack

[francislavoie]

As recently discussed on Slack, our RequestMatcher interface is sometimes insufficient in situations where matchers cannot safely operate because it can only return a boolean. Since matchers make a decision whether or not handlers can run, returning false isn't a secure decision because it may cause handlers that must run (e.g. authentication) to not get run, causing possible bypass of security-critical functionality in the config.

We already had a mechanism for this, which I added in #4346 via putting a variable in Context and reading it back after the matcher ran. That was absolutely a hack and not the best solution. I couldn't think of a better way at the time (lack of Go skills 😅)

To solve this, the better solution is to introduce a new RequestMatcherWithError interface which returns (bool, error), so that an error can be returned to cancel the HTTP request pipeline if the matcher's preconditions aren't met. For now, we can make this optional (so if a matcher implements this interface we call it if possible) until we know all matcher plugins have caught up implementing with this (though unfortunately guaranteeing that is quite difficult and a multi-step process to clean up).