Add a method to collect the DNS names from a certificate

[seanmonstar]

This continues on the work from #65, and adds what seems to have been missing per the review:

• Added a test that a certificate that contains wildcards in its Subject Alternative Name list has the wildcard skipped when returning the list.
• I also changed the behavior for the test invalid_subject_alternative_names. Instead of returning a BadDER error because one name wasn't a valid DNSName, those names are just skipped instead.

Closes #64

[seanmonstar]

r? @briansmith

[seanmonstar]

Looks like CI failures are spurious apt-get errors?

[Geal]

Why skip wildcard names? This is the kind of information I'd like to gather from a certificate. Or is there another way to get it?

[briansmith]

Why skip wildcard names? This is the kind of information I'd like to gather from a certificate. Or is there another way to get it?

I support adding support for wildcards as long as non-wildcard and wildcard names are explicitly distinguished in the type system, e.g. adding a WildcardDnsName type and then having this kind of functionality return an enum GeneralDnsName { DnsName(DnsName), WildCard(WildCardDnsName) }.

[briansmith]

Closing this in favor of PR #103. PLMK if this was the wrong thing to do.

Note: I renamed the "master" branch to "main". Sorry for the inconvenience. This PR has had its base branch updated to "main" but you'll need to deal with the change in your local repo yourself.