feat: Add API keys to memgpt server REST API

[cpacker]

Please describe the purpose of this pull request.

• Remove use of user_id in REST API requests in favor of API keys passed as bearer tokens
• Instead, each user can have API keys (stored in the Tokens table)
  • Format: same as OAI, len 52 string with prefix sk-
• When the client makes a REST API request, they are required to pass an API key as a bearer token, which the server then looks up and "decodes" to a user_id

How should the API keys be generated? TODO

• Currently, when memgpt server is run a new API key for the system user is generated for easy testing
• We still need to figure out a proper way to generate these API keys
• Some options:
  • Password-protected REST API route (POST /create/key)
  • memgpt api-key generate command
  • ^Note how in both cases the "server admin" will be able to generate keys only

% memgpt server
new_key = sk-96905f3b39...0f7bce7d06
INFO:     Started server process [96408]
INFO:     Waiting for application startup.
Writing out openapi.json file
INFO:     Application startup complete.
INFO:     Uvicorn running on http://localhost:8283 (Press CTRL+C to quit)
^CINFO:     Shutting down

How to test

• test_metadata_store.py was updated to test creation/deletion of API keys

Have you tested this PR?

• Partially (have not tested all routes)

[sarahwooders]

lgtm!