refactor: global surfacing of known checksum algorithms

[ndushay]

#140 nicely pulls the "known algorithms" for checksum computation into a constant.

It would be great to have this constant either live in https://github.com/sul-dlss/moab-versioning/blob/master/lib/moab.rb, along with DEFAULT_CHECKSUM_TYPES -- or to find an even better place for constants like this.

Second part: ensuring the KNOWN_ALGORITHMS constant is used not only in this place, but anywhere else checksums are computed in the MV code.

Bonus points if constants like these are configurable.

[atz]

IMO, the constant shouldn't be the thing other parts of code rely on directly, but rather some method invocation that encapsulates the presence of the constant (so that configurability can play a filtering role). Same intent.

[ndushay]

See #119