Fuzzing: Initial commit

[AdamKorcz]

This is a draft for work in progress for adding fuzzing to the shared library.

In case anyone wants to comment, ask or test it out, I share the progress in this PR.

The fuzzers can be built and run by:

git clone https://github.com/AdamKorcz/pkg --branch fuzz1
cd pkg/fuzz
sudo docker build . --tag flux-fuzzing-image

To run a fuzzer, modify and uncomment this line in the Dockerfile:

#RUN /fuzzers/FuzzLibGit2Error

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Cross referencing fluxcd/source-controller#443

[stefanprodan]

Superseded by #212