Add close-primes detection via Fermat's factorization

[aarongable]

Add a new check to GoodKey which attempts to factor the public modulus
of the presented key using Fermat's factorization method. This method
will succeed if and only if the prime factors are very close to each
other -- i.e. almost certainly were not selected independently from a
random uniform distribution, but were instead calculated via some other
less secure method.

To support this new feature, add a new config flag to the RA, CA, and
WFE, which all use the GoodKey checks. As part of adding this new config
value, refactor the GoodKey config items into their own config struct
which can be re-used across all services.

If the new FermatRounds config value has not been set, it will default
to zero, causing no factorization to be attempted.

Fixes #5850
Part of #5851

[jsha]

Basically looks good to me, just comment fixes and a modest proposal about the return interface (take it or leave it).

[jsha]

Left a nit but it's definitely nonblocking.

[beautifulentropy]

Looks fantastic!