Environments for functions. For now this is just the image URL for the
function run container. In the future, Environments will also have
build container image URLs, and perhaps parameters to those
containers.
Create/read/update/delete/list. Versioning will be added later. For
now, only the latest update is stored.
FunctionStore uses fileStore for function code and resourceStore for
the Function's metadata. Environment- and HTTPTrigger- Store are just
thin wrappers on resourceStore.
There is probably a better way to organize this code, maybe with
reflection.