feat: subscriptions

[smyrick]

Fixes #155

Example using a Websocket test client

[codecov]

Codecov Report

Merging #215 into master will increase coverage by 0.06%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##             master     #215      +/-   ##
============================================
+ Coverage     98.36%   98.43%   +0.06%     
- Complexity      171      178       +7     
============================================
  Files            55       57       +2     
  Lines           552      574      +22     
  Branches         96       98       +2     
============================================
+ Hits            543      565      +22     
  Misses            4        4              
  Partials          5        5

Impacted Files	Coverage Δ	Complexity Δ
...ia/graphql/exceptions/InvalidQueryTypeException.kt	100% <100%> (ø)	1 <1> (ø)	⬇️
...expedia/graphql/generator/types/FunctionBuilder.kt	100% <100%> (ø)	13 <3> (+1)	⬆️
.../com/expedia/graphql/hooks/SchemaGeneratorHooks.kt	100% <100%> (ø)	0 <0> (ø)	⬇️
...otlin/com/expedia/graphql/SchemaGeneratorConfig.kt	100% <100%> (ø)	5 <0> (ø)	⬇️
src/main/kotlin/com/expedia/graphql/toSchema.kt	100% <100%> (ø)	0 <0> (ø)	⬇️
...hql/exceptions/InvalidSubscriptionTypeException.kt	100% <100%> (ø)	1 <1> (?)
...dia/graphql/generator/types/SubscriptionBuilder.kt	100% <100%> (ø)	5 <5> (?)
...n/com/expedia/graphql/generator/SchemaGenerator.kt	97.56% <100%> (+0.12%)	17 <0> (ø)	⬇️

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[rickfast]

LGTM