Migrate CLI to run on MemGPT Client

[sarahwooders]

Currently the CLI has its own implementation of functions such as attaching/loading data and messaging agents. We should migrate the CLI code to run on top of the Python Client (both the LocalClient and RESTClient).

REST API Client

If the following environment variables are set, then the CLI will default to using a REST server:

MEMGPT_BASE_URL 
MEMGPT_API_KEY

Instead of using enviornment variables, we can also add these as CLI options:

memgpt run --base-url ... --api-key ... 

Local Client

The local client calls server functions and does not require a running service, so the CLI code can simply be migrated to run the local client code instead. However we need to make sure that the REST/local clients implement the same set of functions with the same arguments/return types.