add support for reading and writing RTCP XR packets

[ystreet]

As specified in RFC 3611.

Supports:

• Loss RLE
• Duplicate RLE
• Packet Receive Times
• DLRR
• Receiver Reference Time

Does not support:

• Statistics Summary
• VOIP Metrics

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.58209% with 77 lines in your changes missing coverage. Please review.

Project coverage is 75.71%. Comparing base (3bef0df) to head (a1d80fd).

Files with missing lines	Patch %	Lines
src/xr/mod.rs	78.26%	20 Missing ⚠️
src/xr/dlrr.rs	82.53%	11 Missing ⚠️
src/xr/rle.rs	92.36%	10 Missing ⚠️
src/xr/duplicate_rle.rs	82.60%	8 Missing ⚠️
src/xr/loss_rle.rs	82.60%	8 Missing ⚠️
src/xr/receiver_reference_time.rs	68.00%	8 Missing ⚠️
src/xr/packet_receipt_time.rs	88.52%	7 Missing ⚠️
src/compound.rs	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #10      +/-   ##
==========================================
+ Coverage   72.68%   75.71%   +3.02%     
==========================================
  Files          15       22       +7     
  Lines        1219     1688     +469     
==========================================
+ Hits          886     1278     +392     
- Misses        333      410      +77     

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