Backport GraphQLUnion to 4.x.x (ExpediaGroup#1234)

[plugin] support android compile targets (ExpediaGroup#1231)

* [plugin] support android compile targets

Default (JVM) build use `compileKotlin`/`compileTestKotlin` tasks. Android builds use variants which has their own targets (e.g. `compileDebugKotlin`). Since our tasks are finalized by the compile tasks we cannot apply it on android builds as default JVM tasks dont exist.

* fix compile dependency

Gradle scans all decorated plugin/tasks/extensions classes to find all the injection points, apply validations etc. This means that we cannot have any method signatures in the plugin code that references compile only dependencies. Simple fix is to move the affected code to a separate file. See: gradle/gradle#8411 for details.

* fix android build dependency

* parameterize android plugin version

[plugin] simplify client handling of custom scalars (ExpediaGroup#1215)

* [plugin] simplify client handling of custom scalars

Instead of generating the wrapper types we can just annotate the target fields with corresponding serializers/converters.

* fix javadoc

Add local schema file support to Gradle plugin extension (ExpediaGrou…

…p#1212)

* Add local schema file support to Gradle plugin

* update new property to schemaFileName

* fix test - use absolute path for schema file

Co-authored-by: Scott Rossillo <[email protected]>

[plugin] better error messages when generating clients (ExpediaGroup#…

…1182)

* [plugin] better error messages when generating clients

Adds operation name to all thrown exceptions so it is easier to find the invalid file.

Resolves: ExpediaGroup#1173

* separate invalid polymorphic selection set exceptions

* fix unit tests

Client Generation - Allow schema to be on the classpath (ExpediaGroup…

…#1136)

* Client Generation - Allow schema SDL to be on the classpath

* Suggestions and Refactors

* Minor touchup

* More improvements to the code, remove redundant edits

Add valid locations annotation (ExpediaGroup#1132)

* Add valid locations annotation

* Update annotation input

* Mark function as interna;

[plugin] fix compile dependency of test client (ExpediaGroup#1124)

`GraphQLGenerateTestClientTask` was extending `GraphQLGenerateClientTask` which meant that we were configuring all task of type `GraphQLGenerateClientTask` we were also configuring test client. Moved the common client generation logic to a new `AbstractGenerateClientTask` which is used by both client generation tasks.

[docs] fix subscription docs and formatting (ExpediaGroup#1121)

[docs] fix subscription docs and formatting (ExpediaGroup#1121)