Agora RTC Objective-C API Reference Refactor

Here is a list of all functions, variables, defines, enums, and typedefs with links to the files they belong to:

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- OPTIONAL_DECLSPEC_EMPTY_BASES : AgoraOptional.h
- OPTIONAL_ENUM_CLASS : NGIAgoraVideoFrame.h , IAgoraLog.h , AgoraRefPtr.h
- OPTIONAL_ENUM_SIZE_T : AgoraMediaBase.h
- OPTIONAL_LOG_LEVEL_SPECIFIER : IAgoraLog.h
- OPTIONAL_NULLPTR : AgoraBase.h
- OPTIONAL_OVERRIDE : NGIAgoraVideoTrack.h
- OPTIONAL_PROCESSRESULT_SPECIFIER : NGIAgoraMediaNode.h
- OPTIONAL_REFCOUNTRELEASESTATUS_SPECIFIER : AgoraRefCountedObject.h