Flexible configuration for secrets and tokens

[wiltsecarpenter]

Database client tokens and Secrets should be definable in a flexible manner including:

• Configuration file, e.g., ~/.observablehq (currently used by database proxy)
• Environment variables
• Command line arguments

Databases and secrets have names and of course there can be more than one in a project, that implies that in a key/value definition, like an environment variable, there will need to be multiple keys. One possible solution is to include the database or secret name in the key, like "DATABASE_TOKEN_mydatabase" or "SECRET_mysecret"