fix: Support serialized JSON environment variables

[baumandm]

Some of the newer AI configs expect a dict, with the assumption that a custom querybook_config.yaml file is mounted into the Docker volume. Unfortunately that's not straightforward in our deployment, so we're exclusively using environment variables. As a workaround we've hardcoded the dict type configs into our Docker image, which is less flexible.

To address this, this PR adds support for deserializing JSON dicts/lists in configs. It handles deserializing for both environment variables and YAML configs, so it should be backwards compatible with existing config files.

An example config:

AI_ASSISTANT_CONFIG='{ "default": { "model_args": { "model_name": "gpt-3.5-turbo", "temperature": 0.2 }}}'

Two configs already expected serialized JSON: FLASK_CACHE_CONFIG and GOOGLE_CREDS. Now they can be specified as either string-serialized JSON (like before) or a dict.

Note: I restricted this to dict and list types to avoid issues with int or bool types, e.g. where Querybook expects a value of "true" not true. I didn't want to go through the entire code base to update all of those.