Stacks Network github actions

cleanup - removes unused packages/dirs from a runner, freeing around 48GB of space on a runner
codecov - Uploads codecov reports with a retry if it fails (optionally it can run grcov to generate a report to send)
openapi - Generates and uploads an OpenAPI artifact
docker - Generic Docker setup workflows
stacks-core/cache - cache actions for the stacks-core repo
stacks-core/testenv - Test environment actions for the stacks-core repo

Why does this exist?

Quite simply: DRY.

Steps used across several workflows were basically all copy/pasted, requiring a search/replace when an update was needed (i.e. updating the version).
There was a specific issue with free disk space on a runner VM, and some steps were added to a workflow to address it - only to have the same issue reoccur in another workflow.
Repeatability and shareability - as more projects are added to the stacks-network org, they can all benefit from using commonly defined composite actions, as opposed to every repo doing it differently (maintenance moves from every repo to a single repo).