Add close/1. Handle DTLS close_notify alert

[mickel8]

This PR:

• adds close/1 that irreversibly closes peer connection. Most of the functions (except getters) will return {:error, :invalid_state} when called after closing peer connection. Calling close/1 will also send close_notify DTLS alert to the other side
• handles incoming close_notify DTLS alert. This alert moves DTLS transport to the closed state but does not move peer connection, signaling or ice transport to the closed state. Peer connection that received close_notify DTLS alert can still do ice restart with a different peer. See: RTCPeerConnection does not move to the closed state when RTCDTLSTransport receives close_notify DTLS alert w3c/webrtc-pc#3053

Resources:

• W3C WebRTC sec 5.5.1

closes elixir-webrtc/ex_ice#78

[codecov]

Codecov Report

Attention: Patch coverage is 60.24096% with 33 lines in your changes missing coverage. Please review.

Project coverage is 87.70%. Comparing base (598dc4e) to head (b3f709e).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/peer_connection.ex	51.28%	19 Missing ⚠️
lib/ex_webrtc/dtls_transport.ex	71.42%	8 Missing ⚠️
lib/ex_webrtc/sctp_transport.ex	57.14%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
- Coverage   88.06%   87.70%   -0.37%     
==========================================
  Files          50       50              
  Lines        2623     2684      +61     
==========================================
+ Hits         2310     2354      +44     
- Misses        313      330      +17     

Files with missing lines	Coverage Δ
lib/ex_webrtc/ice_transport.ex	87.50% <100.00%> (+7.50%)	⬆️
lib/ex_webrtc/rtp_transceiver.ex	91.04% <ø> (ø)
test/support/test_utils.ex	85.71% <100.00%> (ø)
lib/ex_webrtc/sctp_transport.ex	82.32% <57.14%> (-0.92%)	⬇️
lib/ex_webrtc/dtls_transport.ex	84.56% <71.42%> (-0.44%)	⬇️
lib/ex_webrtc/peer_connection.ex	85.62% <51.28%> (-1.04%)	⬇️

... and 1 file with indirect coverage changes

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