Adds a NativeLibraryConfig.WithLogs() overload to let the user indicate the log level (with "info" as the default)

- Made `NativeApi` into a `static class` (it's not intended to be instantiated)
 - Moved `LLamaTokenType` enum out into a separate file
 - Made `LLamaSeqId` and `LLamaPos` into `record struct`, convenient to have equality etc

- Enabled more features in build process (VBMI and VNNI)
 - Added runtime checking for this features
 - Improved runtime checking to no longer require dotnet8.0

- using `Lazy<T>` to initialize it automatically.
 - Added in `AVX512` support for all dotnet versions (but not autodetected).
 - Added in AVX version auto detection.