Support omit and emit of field default values in struct constants

[benjamin-bader]

Java: no change needed in ConstantBuilder; struct builders already include default values
Java: codegen had a bug where we forgot to include a static initializer if only struct constants were present; fixed here.
Kotlin: builder codepath guarded against missing required fields; restriction softened to allow leaving out default-valued fields
Kotlin: builderless codepath now accounts for default values
Schema: Struct validation now allows for missing fields with default values

[codecov-commenter]

Codecov Report

Merging #507 (0a38c90) into master (86a57f3) will increase coverage by 0.54%.
The diff coverage is 71.42%.

@@             Coverage Diff              @@
##             master     #507      +/-   ##
============================================
+ Coverage     59.73%   60.27%   +0.54%     
- Complexity      877      880       +3     
============================================
  Files            70       70              
  Lines          6608     6611       +3     
  Branches       1024     1026       +2     
============================================
+ Hits           3947     3985      +38     
+ Misses         2360     2325      -35     
  Partials        301      301              

Impacted Files	Coverage Δ
.../com/microsoft/thrifty/kgen/KotlinCodeGenerator.kt	85.07% <33.33%> (+0.07%)	⬆️
.../com/microsoft.thrifty.gen/ThriftyCodeGenerator.kt	71.02% <100.00%> (+3.23%)	⬆️
...in/kotlin/com/microsoft/thrifty/schema/Constant.kt	79.61% <100.00%> (+1.23%)	⬆️
...otlin/com/microsoft.thrifty.gen/ConstantBuilder.kt	36.36% <0.00%> (+7.27%)	⬆️

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