Add option to fully disable registry cache (including initial loading)

**Is your feature request related to a problem? Please describe.**
Currently, even if we set `cache_ttl_seconds=0` for the SQL registry, the registry still loads all feast objects into the cache duration initialization. This initial loading can be very time-consuming, especially for large registries.

**Describe the solution you'd like**
Introduce a configuration option (e.g., `cache_enabled=False` or `cache_mode="none"`) to allow users to opt out of any caching, including the initial full load at startup.
With this option, all registry operations would query the backend database directly, and no in-memory object cache would be used.

**Describe alternatives you've considered**

**Additional context**