Add/refactor `set_local_description` and `create_answer`

[codecov[bot]]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
lib/ex_webrtc/peer_connection.ex	70.00% <100.00%> (+45.16%)	:arrow_up:
lib/ex_webrtc/session_description.ex	0.00% <0.00%> (ø)
lib/ex_webrtc/sdp_utils.ex	82.66% <70.00%> (-4.61%)	:arrow_down:

... and 2 files with indirect coverage changes

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