Use Pydantic 2 secrets for `PRIMARY_STORE` quacc setting

[Andrew-S-Rosen]

What new feature would you like to see?

Currently, the `PRIMARY_STORE' quacc setting is stored in the default quacc YAML file. But this isn't ideal because we should keep secrets (eg credentials) elsewhere. Pydantic 2, via pydantic-settings, now supports secret management. We should use that instead.

The same issue exists in https://github.com/materialsproject/jobflow/issues/451.