Add fake credential generator interfaces

[Firstyear]

Fixes #362 - adds a fake credential ID generator so that RP's may choose to implement account enumeration defences.

• [ x ] cargo test has been run and passes
• [ x ] documentation has been updated with relevant examples (if relevant)

[Firstyear]

That's also why there is a trait so that people can make their own distributions :)

[Firstyear]

Waiting on @micolous to review as well :)