Move json alternative parsing away from the main thread

[kried]

Description

Previously, the SDK parsed continuous alternatives each time method show(routeResponse) was called on the main thread. Also, each reroute callback was totally handled on the main thread, including parsing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.05%. Comparing base (4b532fb) to head (0e79905).
⚠️ Report is 12 commits behind head on lts/v2.

@@            Coverage Diff             @@
##           lts/v2    #4729      +/-   ##
==========================================
+ Coverage   60.01%   60.05%   +0.03%     
==========================================
  Files         189      189              
  Lines       21324    21324              
==========================================
+ Hits        12798    12806       +8     
+ Misses       8526     8518       -8     

see 2 files with indirect coverage changes

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