Compiler plugin API + classloader isolation

[martinbonnin]

Description

It's possible to customize the compiler output using one of the many Gradle options. Most of them are simple Strings/Int/Boolean that are easy to serialize and pass through but sometimes, the customization requires code. Two examples are:

• OperationOutputGenerator
• CompilerHooks

In those cases, the apollo-gradle-plugin API exposes a version that can be bumped whenever something changes in the implementation. While working, it's a bit cumbersome.

Instead, we could have a configuration that we load in an isolated worker:

dependencies {
  add("apollo${Service}Compiler", project(":apollo-compiler-plugin"))
}

Not only does it remove the version workaround but it promotes the apollo-compiler API to a first-class API and potentially removes some classloading issues (especially around KotlinPoet)