Remove object store cls

[dakinggg]

What does this PR do?

Removes object_store_cls and instead parses out which remote backend to use given the passed in bucket uri. Also some renaming of related variables.

What issue(s) does this change relate to?

Closes CO-1187
Related to CO-1167

Manual testing

Tested s3 with kwargs

    object_store_logger = RemoteUploaderDownloader(
        remote_bucket_uri="s3://mosaicml-internal-checkpoints-test",
        remote_path_format_string="{remote_file_name}",
        remote_backend_kwargs={"bucket": "mosaicml-internal-checkpoints-test", "prefix": "daniel-test"}
    )

Tested s3 without kwargs

    object_store_logger = RemoteUploaderDownloader(
        remote_bucket_uri="s3://mosaicml-internal-checkpoints-test",
        # remote_path_format_string="daniel-test/{remote_file_name}"
    )

Tested libcloud with s3

    object_store_logger = RemoteUploaderDownloader(
        remote_bucket_uri="libcloud://mosaicml-internal-checkpoints-test",
        remote_backend_kwargs={"provider": "s3", "provider_kwargs": {
            "secret": "----",
            "key": "----",
            "region": "us-west-2"
            }}
    )

Might want to see if I can test GCS too.

[mvpatel2000]

Mostly LGTM. Deferring approval to Evan though

[eracah]

LGTM. Thanks for doing this! This will greatly simplify the checkpointing PR