Add Yamux experimental support

[Menduist]

Mplex is starting to get slowly kicked out of libp2p, and it's probably for the best

We've had experimental yamux in nim-libp2p for a while, but it had limited testing on actual networks
Adding this flag, so that we can test on a few fleet nodes, and in some future, make it default to true

Also see libp2p/specs#553

[tersec]

Maybe mark this hidden, and/or otherwise signal that it's not a supported option?

This seems ideally ephemeral/temporary and to be removed at some point.

[Menduist]

Thanks, I wasn't aware that this was possible

[github-actions]

Unit Test Results

         9 files  ±0    1 071 suites  ±0   33m 53s ⏱️ - 4m 52s
  3 698 tests  - 1    3 419 ✔️  - 1  279 💤 ±0  0 ❌ ±0 
15 760 runs   - 3  15 455 ✔️  - 3  305 💤 ±0  0 ❌ ±0 

Results for commit b3a5ff3. ± Comparison against base commit 939b300.