Logfire API Shim

The `logfire-api` library is a lightweight shim for the Logfire SDK, designed for packages that want to offer opt-in integration with Logfire without a hard dependency. It provides a clone of the `logfire` package's Python API that performs no-op operations if `logfire` is not installed, but makes real calls when `logfire` is present. This allows users of an integrated package to decide whether to install and configure Logfire for observability. The current version is 4.31.0, with frequent releases aligning with the main Logfire SDK.