[Storage Cleaner] Improvements to moving runs

[2015aroras]

This is the first of 3 PRs that make changes to the move mechanism of the storage cleaner. The other PRs will:

1. Add the option to move just 1 entry from a run, making the script more parallelizable.
2. Migrate off TrainConfig to support legacy checkpoints.

This PR makes the following changes:

• Adds most of the functionality for moving non-checkpoint entries when append_wandb_path is set. This will make it possible to upload train_data to all folders that need it.
• Lets the storage adapter choose what happens when moving an entry to a destination where the entry already exists. This change is not necessary but is convenient.

[epwalsh]

LGTM