[experiment][dpg] do not convert enum client parameters into string

[archerzz]

Description

In DPG, we use InputModelTypeUsage.None as a tricky way to convert enum client parameters into string.

autorest.csharp/src/AutoRest.CSharp/LowLevel/AutoRest/DpgOutputLibrary.cs

Lines 123 to 126 in 23dcd81

	if (!_isTspInput || enumType.Usage == InputModelTypeUsage.None)
	{
	return TypeFactory.CreateType(enumType.EnumValueType);
	}

This won't work when adopting TCGC because TCGC will give a InputModelTypeUsage.Input for client parameters. Then enum client parameters will still be enum, which will cause exception when trying to get Field from parameter (which is enum value type):

autorest.csharp/src/AutoRest.CSharp/Common/Output/Models/ClientFields.cs

Line 131 in 23dcd81

public FieldDeclaration? GetFieldByParameter(Parameter parameter)

This PR investigate one solution to resolve this problem. It will change the logic of finding matching field. For enum field type, we'll try to match its value type as well. If that matches, then we'll return that field.

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