Add nullability annotations to APIs commonly used by services

[nicmunroe]

This significantly improves kotlin support.

[codecov-io]

Codecov Report

Merging #119 into master will decrease coverage by 0.19%.
The diff coverage is 94.35%.

@@             Coverage Diff             @@
##             master     #119     +/-   ##
===========================================
- Coverage     91.71%   91.52%   -0.2%     
- Complexity     2348     2373     +25     
===========================================
  Files           161      161             
  Lines          6735     6817     +82     
  Branches        889      916     +27     
===========================================
+ Hits           6177     6239     +62     
- Misses          366      386     +20     
  Partials        192      192

Impacted Files	Coverage Δ	Complexity Δ
...ver/error/validation/RequestSecurityValidator.java	100% <ø> (ø)	1 <0> (ø)	⬇️
...riposte/config/BackstopperRiposteConfigHelper.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...oste/server/handler/SecurityValidationHandler.java	75% <ø> (ø)	15 <0> (ø)	⬇️
...e/server/http/filter/RequestAndResponseFilter.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...c/main/java/com/nike/riposte/util/MatcherUtil.java	100% <ø> (ø)	3 <0> (ø)	⬇️
...oste/metrics/codahale/CodahaleMetricsListener.java	100% <ø> (ø)	40 <0> (ø)	⬇️
...ike/riposte/server/http/impl/BaseResponseInfo.java	100% <ø> (ø)	25 <0> (ø)	⬇️
...e/serviceregistration/eureka/EurekaServerHook.java	100% <ø> (ø)	4 <0> (ø)	⬇️
...erver/handler/RequestContentValidationHandler.java	100% <ø> (ø)	19 <0> (ø)	⬇️
...ava/com/nike/riposte/metrics/MetricsCollector.java	100% <ø> (ø)	12 <0> (ø)	⬇️
... and 43 more

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