Create property delegate function for ThreadLocal

[fractalwrench]

Goal

Neither Gson or EmbraceSerializer are thread-safe. This changeset updates both to use ThreadLocal to avoid any thread safety issues around JSON serialization, and adds some syntactic sugar that makes it easier to use thread locals.

Testing

Added unit test coverage.

[codecov]

Codecov Report

Merging #58 (f6a9ca8) into master (3fbf6ad) will increase coverage by 0.08%.
Report is 3 commits behind head on master.
The diff coverage is 82.35%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   74.58%   74.67%   +0.08%     
==========================================
  Files         307      308       +1     
  Lines       10094    10095       +1     
  Branches     1453     1448       -5     
==========================================
+ Hits         7529     7538       +9     
+ Misses       1986     1982       -4     
+ Partials      579      575       -4     

Files	Coverage Δ
...esdk/comms/delivery/EmbraceDeliveryCacheManager.kt	77.31% <100.00%> (ø)
...embracesdk/internal/utils/ThreadLocalExtensions.kt	80.00% <80.00%> (ø)
...e/android/embracesdk/internal/EmbraceSerializer.kt	63.15% <81.81%> (+26.31%)	⬆️

... and 2 files with indirect coverage changes

[lucaslabari]

LGTM