Support External Account Binding (EAB) tokens to the Manager as defined in RFC 8555, Section 7.3.4. If the ExternalAccountBinding field is set on Manager, pass it into the acme Account during registration. Fixes golang/go#48809 Change-Id: I64c38b05ab577acbde9f526638cc8104d15ff055 Reviewed-on: https://go-review.googlesource.com/c/crypto/+/354189 Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org> Trust: Brad Fitzpatrick <bradfitz@golang.org> Reviewed-by: Filippo Valsorda <filippo@golang.org> Run-TryBot: Filippo Valsorda <filippo@golang.org> Auto-Submit: Filippo Valsorda <filippo@golang.org> TryBot-Result: Gopher Robot <gobot@golang.org>