log4net to Serilog: Log format cleanup / rework

[tippmar-nr]

Fixed some issues with log formatting, particularly in the Audit log output. Added another Serilog package to help with defining the log format similarly to the way we did with log4net.

Modified the various XxxFormat() methods to pass the format string and args through to Serilog, which ensures proper formatting of the string especially when the format string contains embedded JSON.

Also re-worked the guard checks in Logger.cs and Log.cs so that the check is made at the lowest level.

[tippmar-nr]

Waiting on a green CI run before I merge. Not positive I've got all the edge cases handled yet.

[codecov-commenter]

Codecov Report

Merging #1672 (a001672) into feature/log4net-to-serilog (98dfc0d) will increase coverage by 0.13%.
The diff coverage is 83.78%.

@@                      Coverage Diff                       @@
##           feature/log4net-to-serilog    #1672      +/-   ##
==============================================================
+ Coverage                       82.41%   82.55%   +0.13%     
==============================================================
  Files                             389      385       -4     
  Lines                           24180    24151      -29     
==============================================================
+ Hits                            19929    19937       +8     
+ Misses                           4251     4214      -37     

Impacted Files	Coverage Δ
.../NewRelic/Agent/Core/Logging/LogLevelExtensions.cs	96.55% <0.00%> (-3.45%)	⬇️
src/Agent/NewRelic/Agent/Core/Logging/AuditLog.cs	72.72% <66.66%> (-27.28%)	⬇️
src/NewRelic.Core/Logging/Log.cs	84.81% <83.33%> (+0.27%)	⬆️
.../NewRelic/Agent/Core/Logging/LoggerBootstrapper.cs	81.08% <90.90%> (+10.75%)	⬆️
src/Agent/NewRelic/Agent/Core/Logging/Logger.cs	96.25% <100.00%> (+0.25%)	⬆️

... and 5 files with indirect coverage changes

[jaffinito]

Nice work here!