fix lazy import in models.py

[jku]

#1496 moves more ClientTrustConfig and related classes (SigningConfig, TrustedRoot) to models. To accomodate that, lazy imports were added for RekorClient and Rekor2Client. It would be nice to clean this up.

The issue appears because we now import Rekor*Client in models but those want to import TransparencyLogEntry from models...