Support for marshalling optional parameters in `[GeneratedComInterface]` interfaces

[max-charlamb]

Currently GeneratedComInterface supports passing COM interfaces as method arguments using [in] and [out] tags but does not support optional COM interface arguments.

Using the the following IDL definition (found in xclrdata.idl) as an example,

HRESULT GetTokenAndScope([out] mdMethodDef* token, [out] IXCLRDataModule **mod);

this could normally be marshalled using a generated interface with a method signature like

[PreserveSig]
int GetTokenAndScope(out uint token, out IXCLRDataModule? mod);

the marshaller will take automatically convert the interface into the native pointer.

However, this method supports passing mod as a nullptr. In this case no COM interface should be marshalled back to the caller. This pattern is not currently supported and requires falling back to manually marshalling the interface.

Related to #66674

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[AaronRobinsonMSFT]

this could normally be marshalled using a generated interface with a method signature like

Nit: The function should be defined as

[PreserveSig]
int GetTokenAndScope(out uint token, out IXCLRDataModule? mod);