(PUP-1840) Add an sha256 checksum type to Puppet::Util::Checksums

Without this patch applied, Puppet doesn't know how to perform
checksums of files using the SHA-256 algorithm. This precludes Puppet
from being useful on hosts configured for compliance with U.S. Federal
Information Processing Standard (FIPS) 140-2.

The SHA-2 family of algorithms is recommended for use by the
U.S. National Institute of Standards and Technology (NIST) in FIPS
140-2; SHA-256 is the algorithm from this family having the shortest
output.

This patch makes SHA-256 available for use as a checksum algorithm by
other parts of Puppet, alongside MD5 and SHA-1.