Support the required flag on resource properties (Discovery and Swagger)

[clementdenis]

This change is 

[codecov-io]

Codecov Report

Merging #41 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master      #41      +/-   ##
============================================
+ Coverage     82.19%   82.24%   +0.05%     
- Complexity     1845     1855      +10     
============================================
  Files           158      158              
  Lines          6672     6692      +20     
  Branches        880      886       +6     
============================================
+ Hits           5484     5504      +20     
  Misses          866      866              
  Partials        322      322

Impacted Files	Coverage Δ	Complexity Δ
...com/google/api/server/spi/config/model/Schema.java	100% <ø> (ø)	2 <0> (ø)	⬇️
...e/api/server/spi/discovery/DiscoveryGenerator.java	92.11% <100%> (+0.05%)	64 <0> (+1)	⬆️
...oogle/api/server/spi/swagger/SwaggerGenerator.java	91.66% <100%> (+0.03%)	150 <0> (+1)	⬆️
.../api/server/spi/config/ResourcePropertySchema.java	68.18% <100%> (+5.02%)	9 <2> (+2)	⬆️
.../api/server/spi/config/model/SchemaRepository.java	93.82% <100%> (+0.07%)	44 <0> (+1)	⬆️
...ig/annotationreader/ApiAnnotationIntrospector.java	96.47% <100%> (+0.47%)	34 <5> (+5)	⬆️
...nfig/jsonwriter/JacksonResourceSchemaProvider.java	80.35% <100%> (+0.35%)	17 <0> (ø)	⬇️
...rc/main/java/com/google/api/server/spi/IoUtil.java	59.52% <100%> (ø)	14 <0> (ø)	⬇️

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[llbrt]

Reviewed 20 of 20 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

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endpoints-framework/src/test/java/com/google/api/server/spi/config/model/SchemaRepositoryTest.java, line 282 at r1 (raw file):

resrouce

resource

[llbrt]

Reviewed 1 of 1 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved