Add support for HLS/DASH content steering

[yreznik-brightcove]

Content steering is a new feature of both HLS and DASH standards, simplifying the design of multi-CDN systems, redundancy/failover functions, load balancing, and various other traffic optimization applications. A number of players (DASH.js, HLS.js, Apple's AVplayer, etc.) already support this feature.

The relevant specifications are:
[1] HLS Content Steering Specification (v1.2b1) https://developer.apple.com/streaming/HLSContentSteeringSpecification.pdf
[2] DASH-IF CTS Version 0.9.0 – https://dashif.org/docs/DASH-IF-CTS-00XX-Content-Steering-Community-Review.pdf

The ask is: please add support for Content Steering functionality in Exoplayer.

Finer-level detail: if feasible, please include the _HLS/DASH_pathway and _HLS/DASH_throughput parameters in calls to the steering servers. These parameters help servers to react faster in cases of failures or other problems with CDNs.

[sr1990]

Don't know how useful this is for adding the support, but do check out POC using Shaka Packager and Exoplayer:
Dash-Industry-Forum/Content-Steering#17
https://github.com/sr1990/ExoPlayer/blob/804f1505beb37299a1f8862becd4b765c043abdb/ContentSteeringREADME.txt#L146

[tonihei]

We are planning to add support for content steering, but this will likely only happen in 2024.

Note that regular BaseURLs (including priorities and weights) are already supported and can be used for slightly more static "content steering" if required.

Don't know how useful this is for adding the support, but do check out POC using Shaka Packager and Exoplayer

We are always happy to accept high-quality pull requests. So if you are willing to provide support for this feature, we can work on getting PRs merged. However, given that this is a slightly more complex feature, it would be helpful to align on the main design ideas first (e.g. by outlining the plan in a Google Doc). I quickly skimmed your POC, which looks like a good start, but there are a few issues that make it difficult to merge it in its current form and we need to resolve them first (e.g. static DCMCClient instead of class instance and immutable result objects, starting a new Thread instead of using Loader/Loadable etc).

Final note (for both filing the issue and potential future PRs): Please open all new issues and PRs in the media3 repository as indicated in the issue template.

[yreznik-brightcove]

FYI, an example test stream for content steering is now available from DASH-IF: https://reference.dashif.org/dash.js/latest/samples/advanced/content-steering.html
Hope it is useful.

[tonihei]

Closing this issue in favor of the same request in the Media3 issue tracker. (androidx/media#1689)