Generalize the capability for using installable certificate stores

[bskinn]

Right now sphobjinv is hardwired to use certifi.

Turns out, there are other certificate stores that people might want to use; e.g., certifi-system-store.

Some users might also prefer to use system certificates.

Refactor the certificate system so that certifi is no longer an install requirement, and put a config system in place (env variables? entry point? API arguments? All of the above? Kind of a plugin thing...) to let users choose whatever third-party certificates system they want, with the default to use no third-party system, but instead the system-defined one.