testing/plugins: migrate cloud fixtures to use fsspec filesystems

[pmrowla]

Cloud fixtures currently depend on each plugin to implement the cloud/path-like utility methods (read_text/bytes, write_text/bytes, etc) and we do this by using the underlying libraries ourselves directly in dvc plugins (i.e. dvc_s3.tests.cloud uses boto directly). It would be easier to add new functionality if the plugin specified an fsspec filesystem to use, and then the generic dvc.testing.Cloud used the fsspec api calls instead. That way adding new functionality for things like retrieving specific version ID's can be added without having to use the underlying aws/gcs/adlfs libraries ourselves.

Basically, since we already added these things in the underlying fsspec implementations, it doesn't make sense for us to be re-implementing them all again in each plugin's Cloud test fixtures.

This would also let us implement a generic Cloud.remove() (currently we use DVC utils.fs.remove() for removing local files, but have no equivalent for the cloud/remote fixtures)

related: #8850