Agora RTC Objective-C API Reference Refactor

- createAgoraCredential() : AgoraBase.h
- createAgoraRtcEngine() : IAgoraRtcEngine.h
- createAgoraService() : IAgoraService.h
- declareProviderVersion() : AgoraExtensionProviderEntry.h
- getAgoraCertificateVerifyResult() : AgoraBase.h
- getAgoraCurrentMonotonicTimeInMs() : AgoraBase.h
- getAgoraExtensionControl() : AgoraExtensionProviderEntry.h
- getAgoraLicenseCallback() : AgoraBase.h
- getAgoraSdkErrorDescription() : AgoraBase.h
- getAgoraSdkVersion() : AgoraBase.h
- registerProviderEntry() : AgoraExtensionProviderEntry.h
- setAgoraLicenseCallback() : AgoraBase.h
- setAgoraSdkExternalSymbolLoader() : AgoraBase.h