Fixes https://github.com/vector-im/swift-ogg/issues/4

The existing code blindly accepted the sample rate in the opus file header(if it was within a specific range).

However as per the opus documentation:

Sampling rate to decode at (in Hz). This must be one of 8000, 12000, 16000, 24000, or 48000.

To fix the issue I have:

1. Fixed the incorrectly handled optional that was causing a crash before we could read the status error from the attempt to create the decoder. We now throw OpusError.badArgument error if the decoder is nil.
2. Rather than accepting the sample rate in the header, we now choose the closest valid sample rate to that defined in the header(For example in the bug reported the file had an input sample rate of 44100, this now gets mapped to a valid sample rate of 48000).
3. Added a test to verify opus files with invalid sample rates are handled correctly.

Describe the bug As described in https://github.com/vector-im/element-ios/issues/6603, OGGConverter and OGGDecoder make assumptions about the format of the file, in particular that the file is mono. This can result in incorrect decoding and playback when a user tries to play an opus file that wasn't sent using the voice message functionality.

To Reproduce See the issue in Element iOS

Expected behavior The library should determine the format of the audio decode from the file rather than having this hard-coded.

Smartphone (please complete the following information): iPhone 12 on iOS 15.6.1 running Element 1.8.27

1. func testConversionRoundTrip() { let src = Bundle.main.url(forResource: "VoiceRecordingWeb", withExtension: "m4a")!

func testConversionRoundTrip() { let src = Bundle.main.url(forResource: "audio", withExtension: "m4a")!

2. creashed encoder = opus_encoder_create(opusRate, pcmChannels, application.rawValue, &status)
   OGGEncoder.swift:80: Fatal error: Unexpectedly found nil while implicitly unwrapping an Optional value

3. AVAudioRecorder [ AVFormatIDKey: NSNumber(value: kAudioFormatMPEG4AAC), AVNumberOfChannelsKey: 1, AVEncoderAudioQualityKey : AVAudioQuality.low.rawValue, AVLinearPCMBitDepthKey : 16, AVSampleRateKey : 44100.0 ] use this setting to create audio.m4a 文件地址 https://github.com/FSilver/Resource/blob/main/audio.m4a

This PR adds support for converting between opus/ogg and mpeg4aac/m4a files. Most files are opus' own header files. The main files of interest are OGGEncoder and OGGDecoder which were adapted from watson-developer-cloud/swift-sdk. Within that library they were specifcally targeted at the a speed -to-text use case and converted directly to WAV format rather than raw PCM(with no WAV container). This encoder/decoder within this convert directly to linear PCM which is then used in OGGConverted along with apples native AVFAudio and AVFoundation libraries to convert between PCM and mpeg4aac/m4a

Round-trip conversion tests are added in ConverterTests using a an audio recording of 2 minutes(at the time of writing the max supported for element voice messages).

Describe the bug A clear and concise description of what the bug is.

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Expected behavior A clear and concise description of what you expected to happen.

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Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

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// initialize opus encoder
encoder = opus_encoder_create(opusRate, pcmChannels, application.rawValue, &status)

happening when I encode an m4a file. the exception is thrown in OGGEncoder.swift