Add support for JSON coverage report

[asherf]

https://coverage.readthedocs.io/en/coverage-5.2.1/config.html#json

Problem

The ability to produce json formatted coverage reports was added to coverage.py a while ago, pants doesn't expose that ability.

Solution

Add json report type to the CoverageReportType enum

[coveralls]

Coverage remained the same at 0.0% when pulling f4bc8bf on asherf:misc into ff2bec3 on pantsbuild:master.

[Eric-Arellano]

Cool. Are you planning on the change to Coverage being a separate PR? It'd probably be better to include in this PR so that the changelog is smaller for our users.

[asherf]

not planning to change any config in this repo.
Just wanted to expose that functionality since the underlying coverage.py supports it.
I need to add some tests now that the other PR is merged.
so I will be adding logic to existing integration test to cover this use case.

[asherf]

fixed issue calling coverage report with the correct params.
2830210
this should be g2g

[Eric-Arellano]

Thanks Asher!