[builder] Save configuration file as part of the build process

[ebezzi]

We should dump the configuration file used by the builder to the census artifacts. This will make it easier to debug issues with the builder and help reproducibility.

For safety and convenience, use the log (private) bucket: s3://cellxgene-data-public-logs/builder/<TAG>/config.yaml

Should be written by both cellxgene_census_builder/build_soma/__main__.py and cellxgene_census_builder/__main__.py

[bkmartinjr]

should also save the datasets manifest used to seed the build process