Add stdlib::sha256

[jcpunk]

This exposes the sha256 function for computation against random data.

[puppet-community-rangefinder]

stdlib::sha256 is a function

that may have no external impact to Forge modules.

This module is declared in 318 of 580 indexed public Puppetfiles.

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[david22swan]

LGTM

[david22swan]

@jcpunk Thank's for putting in the work