Make DataChannel support optional

[LVala]

Now the user is forced to use ex_sctp and therefore needs to install Rust. This PR makes the ex_sctp dependency and DataChannel support as a whole optional.

[codecov]

Codecov Report

Attention: Patch coverage is 82.55814% with 30 lines in your changes missing coverage. Please review.

Project coverage is 87.58%. Comparing base (e79d429) to head (1940150).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/sctp_transport.ex	82.45%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   87.64%   87.58%   -0.06%     
==========================================
  Files          40       40              
  Lines        2225     2231       +6     
==========================================
+ Hits         1950     1954       +4     
- Misses        275      277       +2     

Files with missing lines	Coverage Δ
lib/ex_webrtc/peer_connection.ex	85.90% <100.00%> (ø)
lib/ex_webrtc/sctp_transport.ex	82.08% <82.45%> (-0.56%)	⬇️

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e79d429...1940150. Read the comment docs.