Implement PBKDF2-HMAC-SHA256 and PBKDF-HMAC-SHA512 password encoding schemes

[lscorcia]

This PR introduces the PBKDF2-HMAC-SHA256 and the PBKDF-HMAC-SHA512 password encoding schemes. They are essentially identical to the existing PBKDF2 encoding scheme, except that the latter uses the SHA1 hashing function, while the former use SHA256/SHA512.

Thanks to this similarity, it is possible to generalize most of the code into an abstract class and the only change required is to choose the hashing algorithm + some minor refactoring.

Preencoded values for testing:

testing
-> {PBKDF2-HMAC-SHA256}15000:lShdzU33covbDNiqGVDffdHh/86VaECJlaaNXchT0exQanVuMVRNR0VRbk0=

During testing it can be useful to tweak the default password policy to allow preencoded passwords with the following command:

./bin/dsconfig -p 4444 -h localhost -D "cn=Directory Manager" -w <password> set-password-policy-prop --policy-name "Default Password Policy" --set allow-pre-encoded-passwords:true -Xn

This PR also adds the corresponding server attributes documentation.

[vharseko]

thanks 👍