Add `Mac` factory API to implement keyed hashing for `BLAKE2`

[05nelsonm]

BLAKE2 is capable of keyed hashing in order to support Mac and PRFs. Public access to constructor parameters keyLength and salt is necessary to add it.

Instead of blurring lines of functionality and purpose between the Digest and Mac abstractions, adding this directly in the BLAKE2Digest (like how Bouncy Castle has done) would be shortsighted. A single function factory abstraction for the final class' companion object (like XofFactory) to use as a hook for a proper Mac implementation would be superior.