Improvements for `gotestsum tool ci-matrix`

[dnephin]

#258 adds a new subcommand gotestsum tool ci-matrix for using the timing in test2json files to split all the packages into buckets of packages that can be run in parallel in github actions.

This issue is to track improvements to the new subcommand:

• Add support for splitting a single package into buckets of tests ci-matrix: add partition-tests-inpackage #277 - useful for integration test packages that contain many slow tests
• Support different output formats. The current format is optimized for integrating directly into github actions workflows. But the logic in this subcommand could also be used with other CI systems, or it might be integrate into existing tooling
• Document gotestsum tool ci-matrix in the README.
• Add a new --jsonfile-timing=<filename> flag to gotestsum as an alternative to --jsonfile. The file would include only the events for run, pass, and fail (no output events). This would significantly reduce the size of the files that need to be stored in artifacts and cache to calculate the partitioning of packages in future runs.