Add support for Audio Message transcripts

[zlshames]

The transcript will be in the attributed body, for example:

         NSAttributedString {
[1]           string: '',
[1]           runs: [
[1]             {
[1]               range: [ 0, 1 ],
[1]               attributes: {
[1]                 __kIMFileTransferGUIDAttributeName: '05F19A0E-C4D4-4F6A-9713-E8BE34C55461',
[1]                 IMAudioTranscription: 'This should be transcribed',
[1]                 __kIMBaseWritingDirectionAttributeName: -1,
[1]                 __kIMMessagePartAttributeName: 0
[1]               }
[1]             }
[1]           ]
[1]         }
[1]       ],