Add JSON support to parse conformance files

[podborski]

During MPEG 135 it was suggested to add more tests based on conformance files from Part 32.

In the first step we could fetch conformance files from mpeg server and check if the reference software can successfully open all of them.

In the next step we should continue improving tests by parsing deeper in the files...

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Edit: we need JSON support for this software. So that we can run the software on the conformance files in https://github.com/MPEGGroup/FileFormatConformance and use the json dumps in that repository.

[dwsinger]

We would also like to start generating a reference-output-set of files, for other implementations to cross-check.

[podborski]

We don't use mpeg conformance server anymore and moved to https://github.com/MPEGGroup/FileFormatConformance completely. Currently that repo utilizes MP4Box but having json output support in this project would also allow us to run this software and use it's json output in https://github.com/MPEGGroup/FileFormatConformance.