mgmt: mcumgr: fs: Add hash/checksum (with CRC32/SHA256) handler

This adds a hash/checksum mcumgr handler to the file management commands
which can be used to get a hask or checksum of a file, and includes
handler implementations for IEEE CRC32 and SHA256.

Signed-off-by: Jamie McCrae <jamie.mccrae@lairdconnect.com>