feat: add password protection to /admin routes on server

[cpacker]

Please describe the purpose of this pull request.

• Puts /admin routes on REST server (used to create users, create API keys, etc) behind bearer token auth by default
• The password (to be passed as a bearer token) can be specified when running the server using env variable MEMGPT_SERVER_PASS
• If the password is not specified, it will be auto-generated (and written to stdout)

How to test

• Start server
• Try using /admin route w/o bearer token, should get 401 Unauthorized
• Try adding the bearer token, it should then work
• Also try specifying the password via the env var

Have you tested this PR?

 % memgpt server
Generated admin server password for this session: jBNDBB...w5_wg

% export MEMGPT_SERVER_PASS="banana"
% memgpt server                     
Using existing admin server password from environment.