Add support for receiving DTMF tones

[mickel8]

This PR adds support for DTMF tones.

It also slightly modifies behavior of rtp receiver. So far, we have been expecting only a single codec, and when we received a packet with different payload type, we were logging this as warning.

Right now, we allow for any codec from transceiver codecs with the following notes:

• if a packet has a payload type that does not belong to the codecs list, it is dropped but stats are updated. This is similar to what chromium does
• if a packet has different clock rate than the clock rate used to initialize RTCP report generators, we log this as a warning but we accept such a packet

Resources:

• https://www.rfc-editor.org/rfc/rfc4733
• https://issues.chromium.org/issues/419726208#comment2

[codecov]

Codecov Report

Attention: Patch coverage is 65.62500% with 11 lines in your changes missing coverage. Please review.

Project coverage is 88.02%. Comparing base (af0ab87) to head (43e7811).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
lib/ex_webrtc/rtp/dtmf/depayloader.ex	57.14%	6 Missing ⚠️
lib/ex_webrtc/rtp_receiver.ex	75.00%	3 Missing ⚠️
lib/ex_webrtc/peer_connection.ex	0.00%	1 Missing ⚠️
lib/ex_webrtc/rtp_transceiver.ex	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   88.33%   88.02%   -0.31%     
==========================================
  Files          49       50       +1     
  Lines        2598     2623      +25     
==========================================
+ Hits         2295     2309      +14     
- Misses        303      314      +11     

Files with missing lines	Coverage Δ
lib/ex_webrtc/rtp/depayloader.ex	100.00% <100.00%> (ø)
lib/ex_webrtc/rtp/g711/depayloader.ex	100.00% <ø> (ø)
lib/ex_webrtc/rtp/g711/payloader.ex	100.00% <ø> (ø)
lib/ex_webrtc/rtp/opus/depayloader.ex	100.00% <ø> (ø)
lib/ex_webrtc/rtp/opus/payloader.ex	100.00% <ø> (ø)
lib/ex_webrtc/peer_connection.ex	86.65% <0.00%> (ø)
lib/ex_webrtc/rtp_transceiver.ex	91.04% <75.00%> (-0.46%)	⬇️
lib/ex_webrtc/rtp_receiver.ex	76.92% <75.00%> (-1.13%)	⬇️
lib/ex_webrtc/rtp/dtmf/depayloader.ex	57.14% <57.14%> (ø)

... and 1 file with indirect coverage changes

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