move writeKey from auth header to batch body

[wenxi-zeng]

• move writeKey from auth header to batch body
• add instanceId to context object

[codecov-commenter]

Codecov Report

Base: 78.14% // Head: 78.10% // Decreases project coverage by -0.04% ⚠️

Coverage data is based on head (91a6b19) compared to base (7fc55c4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #121      +/-   ##
============================================
- Coverage     78.14%   78.10%   -0.05%     
+ Complexity      443      440       -3     
============================================
  Files            64       64              
  Lines          5761     5759       -2     
  Branches        725      725              
============================================
- Hits           4502     4498       -4     
  Misses          693      693              
- Partials        566      568       +2     

Impacted Files	Coverage Δ
...va/com/segment/analytics/kotlin/core/HTTPClient.kt	81.53% <ø> (-1.08%)	⬇️
...m/segment/analytics/kotlin/core/HTTPClientTests.kt	88.37% <ø> (-0.52%)	⬇️
...tics/kotlin/core/platform/plugins/ContextPlugin.kt	89.47% <100.00%> (+1.23%)	⬆️
...om/segment/analytics/kotlin/core/AnalyticsTests.kt	72.33% <100.00%> (+0.07%)	⬆️
.../analytics/kotlin/core/compat/JavaAnalyticsTest.kt	66.78% <100.00%> (+0.11%)	⬆️
...tics/kotlin/core/platform/plugins/LogTargetTest.kt	70.65% <0.00%> (-1.09%)	⬇️
...n/java/com/segment/analytics/kotlin/core/Events.kt	45.69% <0.00%> (-0.54%)	⬇️

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