Enable Semantic kernel support

- Made all the mechanics of grammar parsing (GBNFGrammarParser, ParseState) internal. Just call `Grammar.Parse("whatever")`.
 - Added a `GrammarRule` class which validates elements on construction (this allows constructing grammar without parsing GBNF).
   - It should be impossible for a `GrammarRule` to represent an invalid rule.

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`