add pbkdf2 hash function for FIPS 140-2 support

[kroepke]

Description

Adding a new password hash function for use with FIPS 140-2 deployments.

Motivation and Context

If enabled through setting user_password_default_algorithm to pbkdf2 the only hash function for user passwords is PBKDF2WithHmacSHA512 which is the recommend one to use.
The iterations default to 10000, which is rather slow. This can be customized with user_password_pbkdf2_iterations.

The stored hash remembers which iteration number was used, however old hashed passwords are no longer usable and must be reset (this is true for bcrypt as well, so there's no new behavior introduced).

How Has This Been Tested?

Unit test.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.