[3.x.x][client] Remove duplicate header (ExpediaGroup#867)

* Remove duplicate header

Copy changes over from ExpediaGroup#865 to `3.x.x` branch

* Update GraphQLClient.kt

* Update GraphQLClient.kt

Remove duplicate Accept header: application/json (ExpediaGroup#865)

[build] revert Kotlin version to 1.3.72 (ExpediaGroup#858)

We are hitting some library and plugin issues with incompatibilities between Kotlin versions. Downgrading back to 1.3.72 for now, we will revisit update to 1.4 sometime in the future.

[client] update ScalarConverter to accept any objects (ExpediaGroup#819)

* [client] update ScalarConverter to accept any objects

Custom GraphQL scalars can be serialized as any of the other built-in scalars (i.e. int, float, etc). Changed method signatures to accept `Any` when converting to scalar, and produce `Any` when generating JSON value.

* update docs to match change to any

Include java getters for ignore checks (ExpediaGroup#842)

Co-authored-by: Shane Myrick <[email protected]>

[generator] Nullability additional input checks (ExpediaGroup#829)

* Add additional input test

* Add required list check

* Fix linter

Co-authored-by: Shane Myrick <[email protected]>

Add new hook to override the isValidAdditionalType check (ExpediaGrou…

…p#826)

* Add new hook to override the isValidAdditionalType check

* Add filter method locations where we add other additional types

Co-authored-by: Shane Myrick <[email protected]>

[generator] Do not add interfaces for input additional types (Expedia…

…Group#821)

* Generate unique input additional types

* Do not add additional type if they are input and interfaces

* Simplify isValidAdditionalType

Co-authored-by: Shane Myrick <[email protected]>

[generator] Update additional types to work with input types (Expedia…

…Group#817)

* Update additional types to work with input types

* Add more test coverage

* Update build.gradle.kts

* Move AdditionalType class to internal

* Remove 4.0.0 comments

Co-authored-by: Shane Myrick <[email protected]>

[docs] cut documentation version for 3.x.x releases (ExpediaGroup#807)

As we prepare to start working on v4.x.x features on master we are cutting current documentation version