Implement Cryptex header extension encryption

[JonathanLennox]

As defined in draft-ietf-avtcore-cryptex-08

[codecov]

Codecov Report

Merging #29 (18f2390) into master (88a9c38) will increase coverage by 3.06%.
The diff coverage is 84.07%.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #29      +/-   ##
============================================
+ Coverage     49.88%   52.95%   +3.06%     
- Complexity      181      220      +39     
============================================
  Files            23       23              
  Lines          1361     1456      +95     
  Branches        256      274      +18     
============================================
+ Hits            679      771      +92     
- Misses          600      604       +4     
+ Partials         82       81       -1     

Impacted Files	Coverage Δ
src/main/java/org/jitsi/srtp/SrtpPolicy.java	63.41% <80.00%> (+7.85%)	⬆️
...rc/main/java/org/jitsi/srtp/SrtpCryptoContext.java	79.27% <82.29%> (+3.63%)	⬆️
...ain/java/org/jitsi/srtp/utils/SrtpPacketUtils.java	77.58% <100.00%> (+16.71%)	⬆️

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