Check speech-recognition support

Receive the capability JSON through CallbackProxy, then validate the format, sample rate, duration, and size:

CallbackProxy *callback = [[CallbackProxy alloc]
    initWithOnSuccess:^(NSString *data) {
        [self applySpeechToTextCapabilitiesJSON:data];
    }
    onFailure:^(NSInteger code, NSString *message) {
        [self disableTranscription];
    }];

Pass this callback to Open_im_sdkSpeechToTextCapabilities. The result contains format, sampleRateHz, maxRecordTimeMs, maxFileSize, provider, requestType, and crossDomain. Do not guess the limits when the call fails.