Adds a `ResolveNSID` method to `identity.BaseDirectory`. I think keeping DNS TXT resolution stuff together in the same package makes sense, even if this is a "lexicon" thing, not really an "identity" thing. Could consider renaming that method to `ResolveLexiconNSID` or similar, to clarify it is about NSIDs in the context of Lexicons, not "NSIDs in general" (though they are currently only defined in terms of Lexicons). Adds a new `ResolvingCatalog` implementation of the `lexicon.Catalog` interface, which does resolution from the live network and adds to a local cache. Also `lexicon.ResolveLexiconData` low-level helper which goes from NSID to Lexicon record data.