Add support for subscriber offer (requires specific version of Janus)

[lminiero]

Unfortunately the latest version of the WHEP draft removed support for server-originated SDP offers, and now forces all servers to accept offers from subscribers instead, leaving servers to generate an answer. This completely breaks compatibility with the Streaming plugin in Janus, which is used for WHEP support in this server and only supports server-originated offers.

I completely disagree with this choice, but my opinion doesn't matter, so I was forced to modify the Streaming plugin to start adding support for this new ugly mode too. This support is currently provided in a new PR, which means that if you want to test this feature, you'll have to compile Janus from the related branch, and not master.

When you do, you can test the offer-mode by passing a offer=true query string attribute to the watch.html page, e.g.:

http://localhost:7090/watch.html?id=test&offer=true

When offer=true is passed, the viewer page will prepare an offer instead of expecting one, and the WHEP server will handle it accordingly. Again, this will only work if the Janus server the WHEP server is using supports that mode: if you use other versions of Janus, it simply won't work.

Please also test this with other WHEP viewers you may have, if any. My simple-whep-server has not been updated yet, and I don't know when I'll do that (if I'll do it at all). Feedback would be welcome.

[lminiero]

PS: notice that this PR doesn't include any support for the new SSE (Server Side Events) functionality the draft added in the new version. While I may add that in the future, I don't have immediate plans to do that, especially considering that functionality may change considerably in upcoming versions, and hasn't really been discussed much.

[lminiero]

Merging, since the feature was merged in Janus too. I'll keep SSE in a PR, though, as that will need to keep track of updates in the spec.