Add support for -set_serial

Hi,

we are heavily using cfssl internally and are facing an issue with one our intermediate CAs currently.
This lead to a situation where we need to create a new intermediate certificate with the same Serial Number as our old one.

This is possible using the -set_serial option of "openssl req".

https://www.openssl.org/docs/man1.0.2/man1/openssl-req.html

What do you think about this feature?
Is it even possible to implement this here?

From my perspective this is quite a big edge case and also far from a recommended thing to do.