Add billing engine

[dwilkie]

Todo

• Run CGRates as a sidecar container
• Disable HTTP
• Disable websockets
• Disable JSON server
• Configure FreeSWITCH
• Use ngrep to check commands to cgrates

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.36%. Comparing base (3aa550b) to head (828ad06).
⚠️ Report is 112 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1270      +/-   ##
===========================================
+ Coverage    98.72%   99.36%   +0.64%     
===========================================
  Files          184       65     -119     
  Lines         3125      939    -2186     
===========================================
- Hits          3085      933    -2152     
+ Misses          40        6      -34     

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