Add RIST protocol support.

[winlinvip]

RIST is a protocol similar to SRT, see https://www.rist.tv/ for detail.

SRS is happy to support both SRT and RIST because we're an open-source community and open to any new protocols, even if they have some bugs or issues.

We support HLS and DASH, even though I personally think DASH isn't great for live streaming. But we still support it for a small group of people or for testing purposes.

As an open-source project, we should support new protocols to help them become popular. SRS's mission is to make live streaming easy for everyone, and we're open to any new protocol.

We're not RIST experts, so we can't support it unless someone sends us a patch for SRS. If you know RIST well and can send us a patch, we'd be happy to review and add it to SRS.