use codecov actions to upload instead of the cli

[matt-codecov]

tl;dr:

• use regular codecov actions to upload coverage and test results, not the cli
• use a matrixed job to upload coverage/junit files to each of our environments concurrently

worker, shared, and API run their tests inside containers, but those containers all bindmount the working directory into the container. so when the tests write junit/coverage files, those files are visible outside of the container as well. this PR uploads those files with upload-artifact

a separate job uses download-artifact + a matrix to submit up to four uploads ({unit, integration} {coverage, junit}) to codecov in each of our four environments

run-tests-split.yml presents an extra complication. the tests are themselves run in a matrix and produce several output files. i've done a few things to deal with that:

• dynamically build the matrix for the test job based on the passed-in number of groups to split into, so now you can actually use the workflow with different split sizes
• assumed that each test job will include its group number in its output filenames. so for example: unit.4.coverage.xml, integration.2.junit.xml
• added the group index to the uploaded artifacts' keys as well, and used the pattern argument for download-artifact to download them all in one go
• provided globs to the files arg in the action, which i think will work

[matt-codecov]

okay after pushing a new release of the test results action (codecov/test-results-action#112) and then yanking a bad release of the CLI (https://pypi.org/project/codecov-cli/10.1.0/) this appears ready to go

[trent-codecov]

LGTM