cov: add function coverage option

[karineek]

I added a new option: --gcov_functions to generate instead of line coverage, the function coverage when using JSON format. I did testing to check that without the new flag, the tool still generates line coverage, and with the new flag, the tool generates function coverage.

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[karineek]

@googlebot I signed it!

[afd]

@karineek I think you need to fix some formatting errors, using the check_all.sh and fix_all.sh scripts under gfauto. Can you give that a try? See "## Presubmit checks and protobuf generation" in the README.