Create OID package

[pgporada]

There are many OIDs spinkled throughout boulder such as the following snippet. We should build a package that defines OIDs by class similar to how pyca/cryptography handles them.

//ca/crl.go: asn1.ObjectIdentifier{2, 5, 29, 28}
//cmd/ceremony/cert.go: var oidOCSPNoCheck = asn1.ObjectIdentifier{1, 3, 6, 1, 5, 5, 7, 48, 1, 5}
//wfe2/wfe.go: asn1.ObjectIdentifier{2, 16, 840, 1, 101, 3, 4, 2, 1}
etc

[mcpherrinm]

We may be able to use the new x509.OID type instead of making a new one: golang/go#60665

[aarongable]

I think we should use both the new x509.OID and the existing asn1.ObjectIdentifier types in our package, since the OIDs we care about fall into both categories (policy identifiers and extension identifiers, respectively).