Add Dolby AC-4 audio channel configuration to DASH manifest parser

Dolby AC-4 audio, used in ATSC-3 broadcasts, includes an alternate AudioChannelConfiguration schemeIdUri that is currently not parsed, for example:

```

```

This is defined (sort of) in DASH-IF IOP for ATSC-3 section 5.4.4.2.1, which references [TS 103 190-2 - V1.2.1 - Digital Audio Compression (AC-4) Standard](https://www.etsi.org/deliver/etsi_ts/103100_103199/10319002/01.02.01_60/ts_10319002v010201p.pdf), section G.3.1.

It mostly maps to "urn:mpeg:mpegB:cicp:ChannelConfiguration" (which is still the recommended scheme), but dolby.com:2015 adds a poorly-defined object-based audio option, which I guess lets the A/V receiver figure out how to stream the audio to the available surround channels.

I'm not sure if I can provide a test stream; I've seen this scheme used in live ATSC-3 signals, but mainly in Dolby MS12 test streams, which I'm not able to share.