Add support for SHA-256

[paulmillar]

RFC 3230 support various checksum algorithms, as registered by IANA.

One checksum algorithm used by various communities is SHA-256.

It would be helpful if gfal were to support transferring data, using HTTP-TPC, where the data's integrity is ensured by comparing the destination file's SHA-256 hash against a known-good value.

For reference, dCache support SHA-256 since v7.2, with v9.2 being a commonly deployed version.

However, please note that, by default, dCache will only calculate SHA-256 when explicitly requested by the HTTP-TPC client.

Please note that support for #24 may be needed to ensure the service calculates the desired SHA-256 checksum value.