Add RTP component as submodule after re-factoring

[moninom1]

Issue #, if available:

What was changed?

• Add RTP component as external project, and update RTP code to call APIs of component.

Why was it changed?

• To apply RTP re-factoring component into dev branch. RTP component is separated out as independent library.

How was it changed?

• Add RTP component as external project, import it via ExternalProject_Add, and update RTP code to call APIs of component.

What testing was done for the changes?

• Verified master demo on Ubuntu
• Verified serialization & deserialization with webrtc_client_test and pass all test cases.
• Verified packetization & depacketization with unit test framework in RTP component

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.66667% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 88.65%. Comparing base (a28e1e4) to head (74afb60).

:exclamation: Current head 74afb60 differs from pull request most recent head f0fc3e1. Consider uploading reports for the commit f0fc3e1 to get more accurate results

Files	Patch %	Lines
src/source/Rtp/RtpPacket.c	96.42%	2 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1975      +/-   ##
===========================================
+ Coverage    88.60%   88.65%   +0.05%     
===========================================
  Files           47       48       +1     
  Lines        12697    12713      +16     
===========================================
+ Hits         11250    11271      +21     
+ Misses        1447     1442       -5     

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