* Support for async resolvers

Solves issue: https://github.com/kobylynskyi/graphql-java-codegen/issues/24

* https://github.com/kobylynskyi/graphql-java-codegen/issues/24 Refactors and fixes

* Renamed asyncApi to generateAsyncApi in MappingConfig
* Changes for adding java.util.concurrent import when it comes to async Query or Mutation
* Improved tests coverage

The GraphQL specification says that Float type is double-precision.
It is thus effectively equivalent to Java's float.

Resolves:
https://github.com/kobylynskyi/graphql-java-codegen/issues/22Co-authored-by: NJoffrey Bion <joffrey.bion@booking.com>

* Add default values generation

* Avoid using List.of() to keep java 8 support
Co-authored-by: NJoffrey Bion <joffrey.bion@booking.com>

Subscription methods with custom return type #14

# Conflicts:
#	src/main/java/com/kobylynskyi/graphql/codegen/model/MappingConfig.java
#	src/test/java/com/kobylynskyi/graphql/codegen/model/MappingConfigTest.java

* Introduce model annotations (for Lombok) #10

* Unit tests for MappingConfig

* Remove unused import in MappingConfig

* Remove code related to modelAnnotations

* Handle DirectiveDefinition in GraphQLDefinitionType determiner

* Adding a catch handler in case there are other types we can't handle

* Fixing typo from last commit