crypto/x509: add new OID type and use it in Certificate

Fixes #60665

Change-Id: I814b7d4b26b964f74443584fb2048b3e27e3b675
GitHub-Last-Rev: 693c741c76e6369e36aa2a599ee6242d632573c7
GitHub-Pull-Request: golang/go#62096
Reviewed-on: https://go-review.googlesource.com/c/go/+/520535


TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Mateusz Poliwczak <mpoliwczak34@gmail.com>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Roland Shoemaker <roland@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>