ModelsParams record class

- Minor changes to cleanup some of the compiler warnings

- Equality, hashing etc all implemented automatically
 - Default values are defined in just one place (the properties) instead of the constructor as well
 - Added test to ensure that serialization works properly

- Deleted `NativeInfo` (internal class, not used anywhere)

- Passing encoding in the `IModelParams`, which reduces how often encoding needs to be passed around

- Converted `SafeLLamaContextHandle` to take a `ReadOnlySpan` for Eval, narrower type better represents what's really needed

The biggest single change is renaming `LLamaModel` to `LLamaContext`