Distinguish between internal and public facing headers and APIs

[kirklandsign]

Right now we don't know whether a c++ header API is for ET internal use or for public developers.

For example, if I write some executor_runner app, and I want to use kernel registry related API:

executorch/runtime/kernel/operator_registry.h

Lines 225 to 248 in cb3eba0

	bool registry_has_op_function(
	const char* name,
	Span<const TensorMeta> meta_list = {});
	/**
	* Returns the operator with a given name and TensorMeta list, if present.
	*/
	::executorch::runtime::Result<OpFunction> get_op_function_from_registry(
	const char* name,
	Span<const TensorMeta> meta_list = {});
	/**
	* Returns all registered kernels.
	*/
	Span<const Kernel> get_registered_kernels();
	/**
	* Registers the provided kernels.
	*
	* @param[in] kernels Kernel objects to register.
	* @retval Error::Ok always. Panics on error. This function needs to return a
	* non-void type to run at static initialization time.
	*/
	ET_NODISCARD Error register_kernels(const Span<const Kernel>);

I have some questions:

• Should I include <runtime/kernel/operator_registry.h> at all? Is it for internal use and I should not use it in my code?
• Say get_registered_kernels() is a getter, and I can use in my app; but register_kernels() is definitely for ET internal use (or for users to set in their codegen/library, not app). Should we distinguish?