Add foundation for independent parsing of junit based output for CI reporting to cassandra-builds

PR attached.

For doing CI ourselves, it's useful to have a single pane of glass report where you have a summary of results for all your suites as well as inlined failures. This should be agnostic to any xunit based output; so long as we co-locate the xunit data in directories adjacent to one another, the script in the PR will generate an in-memory representation of the xunit results as well as inline failures to an existing .html file.

The contents will need to be tweaked a bit to generate the top level branch + sha + checkstyle + summaries information, but the vast majority of that is already parsed and easily available within the script and can be extended pretty trivially.

Opening up a pr to pull this into [cassandra-builds](https://github.com/apache/cassandra-builds) since mck is actively working on that and needs these primitives. I'd expect the contents in ci_parser to be massaged to become a more finalized, full solution before we start to use it but no harm in the incremental merge.