Make `OpenApiAnnotationProcessor` support incremental Gradle builds

I'm getting a warning in the Gradle output of my build:
```
Full recompilation is required because io.javalin.openapi.processor.OpenApiAnnotationProcessor is not incremental.
```
Not high priority, but would be nice to have incremental build support for the annotation processor. Would especially help if there are a large number of `OpenApi` annotations.
Would also be nice to have an option to disable pretty printing for smaller OpenApi spec files ([relevant code](https://github.com/javalin/javalin-openapi/blob/414aff9a6938b49fe5775969f5a43cbafb84d223/openapi-annotation-processor/src/main/kotlin/io/javalin/openapi/processor/generators/OpenApiGenerator.kt#L260)).

Start by inspecting OpenApiAnnotationProcessor and the Gradle annotation-processing requirements for incremental support. Then review openapi-annotation-processor/src/main/kotlin/io/javalin/openapi/processor/generators/OpenApiGenerator.kt around line 260 for the pretty-printing request. Done means the processor no longer triggers full recompilation and generated OpenAPI output can disable pretty printing.

Written by the indexing model from the issue text.