Add settings module

[soapy1]

Fixes #1005

Description

This PR introduces a settings module for interacting with conda store settings. It also adds some documentation and enforces rules for how settings get merged at each of the different levels of abstraction that setting apply to.

There are multiple levels which settings config can be applied to conda store.

• deployment defaults (controlled by the config file)
• global defaults (controlled by the admin console)
• namespace defaults (controlled by the admin console)
• environment default (controlled by the admin console)

The most specific settings config will take precedence over the more general setting.

The introduction of this settings object will allow for plugin authors to have better patterns for accessing settings. For example, consider the lock plugin which requires running a conda command

def _conda_command(self, conda_store) -> str:
        with conda_store.session_factory() as db:
            settings = conda_store.get_settings(db=db)
        return settings.conda_command

can now become

def _conda_command(self, conda_store, namespace=None, environment=None) -> str:
        return conda_store.get_setting(
            "conda_command", namespace=namespace, environment=environment
        ) or "mamba"

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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