VersionAlert for Unity

Callbacks for retrieving the ONNX metadata at import time.
`Upload` method to `Tensor` to allow for direct upload of data.
`IndexSelect` functional operator.
`scoreThreshold` parameter for `NMS` functional method.
`Tensor.Reshape` accepts tensor shapes of different lengths, provided the capacity of the tensor data is large enough.
`Worker.CopyOutput` method for copying outputs into allocated tensors.
`FunctionalTensor.ToString` shows the data type and shape (if known) for debugging when using the functional API.

Made a generic `Tensor` class so that you can declare `Tensor` and `Tensor` rather than `TensorFloat` and `TensorInt`.
Reworked and unified the `Worker` methods for setting inputs and scheduling.
`IWorker` and `GenericWorker` to be an instantiable `Worker` class.
Renamed `Execute` to `Schedule` throughout for clarity.
Renamed `BurstTensorData` to `CPUTensorData` for consistency.
Renamed and reworked `SymbolicTensorShape` as `DynamicTensorShape` for clarity and ease of use.
Reworked the `Functional.Compile` method as methods on a `FunctionalGraph` object, for clearer declaration of inputs and outputs.

`GPUCommandBuffer` backend, in favour of a unified GPUCompute backend using command buffers.
`IBackend` methods, use the functional API to create and edit models instead.
Many methods and classes from the public API to keep it clear and maintainable.

com.unity.sentis 2.0.0