Add an update prompt when using the CLI

[Viicos]

Description

As per our EU region checklist, a follow up feature was to implement a version check when using the CLI (at least for logfire auth, might apply to other commands), similar to what pip does.

[Viicos]

Unfortunately the PyPI API doesn't support querying only for the latest version (see pypi/warehouse#4663). The current payload at https://pypi.org/pypi/logfire/json takes 166.74 kB, which could slown down the CLI on slow connections. Alternatively, we could only check for new versions every x days/weeks (pip does it every week), but this requires some extra work to cache the last check timestamp.

[alexmojaki]

Related: https://pydantic.slack.com/archives/C05AF4A4WRM/p1716821490869969