use Abseil OStringStream in WebGPU EP string concat

[fs-eire]

Description

use Abseil OStringStream in WebGPU EP string concat.

Allows multiple calls to shader_helper.MainFunctionBody()

[skottmckay]

Can we add something like this to decay const char[] to const char*? It saves significant binary size to do so.

onnxruntime/include/onnxruntime/core/common/make_string.h

Lines 91 to 103 in 1bda91f

	// We need to update the types from the MakeString template instantiation to decay any char[n] to char*.
	// e.g. MakeString("in", "out") goes from MakeString<char[2], char[3]> to MakeStringImpl<char*, char*>
	// so that MakeString("out", "in") will also match MakeStringImpl<char*, char*> instead of requiring
	// MakeStringImpl<char[3], char[2]>.
	//
	// We have to do the type processing before any actual work, so this function purely implements the type processing.
	// If we do not do it this way we do not get the full binary size reduction.
	//
	// See https://stackoverflow.com/a/29418212/684911 for overall details of the approach, but note it does not cover
	// the need to do the type processing as a separate step.
	return detail::MakeStringImpl(detail::if_char_array_make_ptr_t<Args const&>(args)...);
	}