Support multiple certificate pairs

It may be useful for a single packager instance to sign for multiple domains that are authenticated by different certs. (This should be an uncommon case, though, as it is very simple and possibly more secure to run different instances each with access to only one certificate.)

No files, tests, or entry points are named in the issue. Start by locating the packager's certificate and signing configuration, then determine how domain-to-certificate selection should work; done means one instance can sign multiple domains with different certificate pairs.

Written by the indexing model from the issue text.