Configuration validation using pydantic

We should validate the configuration files using pydantic, and generate documentation with pydantic-autodoc.

As a bonus, we could implement a command line option that would generate a configuration files with the default values + the environment values.

https://docs.pydantic.dev/latest/usage/settings/

Questions on Flask+pydantic_settings integration:

• How to put a BaseSettings as a app.conf? The object interface is very convenient for development, Flask excepts a dict here.
• A custom BaseSettings won't necessarily cary all the Flask configuration options, so model_config = SettingsConfigDict(extra='allow') might be needed.

Todo

• write configuration using pydantic
• parse configuration with pydantic at startup
• find a way to keep reading secrets from files
• implement, test and document configuration using environment variables
• refactor the existing configuration checks (for example the code that checks that the LDAP connection is valid etc.)
• write a command that generates a configuration file (for example, exports a toml file from the current configuration, including the comments)
• refactor Canaille so it uses pydantic object for Canaille specific configuration (maybe in another MR)