Fix: add gateway name to model cache identifier

[treysp]

Model definitions are cached by the SQLMesh loader. Currently, the cached model ID consists of the latest time any project files were modified, a fingerprint of the context config object, and the context's default catalog.

If a model uses the @gateway macro variable, its value will be stored in the model's python environment and included in the cache. If the project is run with a different gateway, a cache hit can occur with the wrong gateway value being rendered into the model.

This PR fixes this by adding the gateway name to the model cache ID.

Implications:

• All project models will have a cache miss when the gateway changes. The miss will be unnecessary for all models that do not use the @gateway macro variable.
• An alternative approach is to leave the cache ID as-is, and after cache read check whether the gateway has changed and the @gateway macro variable is used. That approach will result in unnecessary cache hits when the gateway has changed and is used in the model.