Re-think logging

[woodruffw]

Per discussion in theupdateframework/python-tuf#2243: our logging behavior is currently as follows:

1. By default, we emit INFO and higher logs
2. We respect SIGSTORE_LOGLEVEL to set the root logger, e.g. SIGSTORE_LOGLEVEL=debug sets DEBUG for all loggers, not just first-party ones.

(2) is arguably incorrect -- SIGSTORE_LOGLEVEL should probably only affect loggers under the sigstore logging hierarchy, not the entire runtime.

Additionally, we should probably support a -v/--verbose flag with additive functionality:

1. -v: equivalent to DEBUG for first-party code (i.e. sigstore.*)
2. -vv: equivalent to DEBUG for all code (i.e. DEBUG on the root logger)

cc @jku