Scala Support: Add a scala compiler plugin

Scala doesn't currently support Java's Annotation Processing API, instead they have a compiler plugin API.

The Micronaut team has no Scala experts as committers, this issue is here to track the fact this is a desired feature, that we are currently looking help for from the Scala community to implement.

For reference Groovy doesn't support APT either, only Kotlin and Java do. The Groovy implementation can be found at:

• https://github.com/micronaut-projects/micronaut-core/blob/master/inject-groovy/src/main/groovy/io/micronaut/ast/groovy/InjectTransform.groovy

The Java/Kotlin implementation at:

• https://github.com/micronaut-projects/micronaut-core/blob/master/inject-java/src/main/java/io/micronaut/annotation/processing/BeanDefinitionInjectProcessor.java

Any takers?