checksumsink: Add parameter to print the checksum of the complete output

The idea behind this PR is to improve the performance of the GStreamer pipeline used by Fluendo Fluster.

Fluster is an open software testing framework for decoders conformance: https://github.com/fluendo/fluster/

Currently, Fluster is using post-processing to get the hash for the output generated by filesink. But with the patch the hash could be retrieved directly from the pipeline. Avoid writing a temporal file.

I update the behavior of the checksumsink element instead of creating a new sink element. But a new sink could be written if it is more clean.

ping @pamarcos