* Enhance the AppConfig with external config sources.
The external config sources are currently environment variables
and AWS secrets manager.
The config file can be augmented with a section describing how
environmen variables and secrets can update config parameters.
benefits:
- it will enable the config to draw from more than one secret. This is useful
for shared secrets between cellxgene and data portal, as well as auth0 secrets.
- it will make it very straightforward to check the config before a deployment.
Part of #1859
* split out config
* add tests for base and app config, refactor client config out of app config
* refactor default config retrieval
* create config test class and helper functions
* move default_config into server to fix import issue
