Support Kotlin multiplatform builds

I'd like to combine separate Gradle projects for generation of Android client protobuf Java (lite) sources and server protobuf Java sources into a single project via a Kotlin multiplatform Gradle project. Clearly the intention of the protobuf Gradle plugin is to generate sources for different languages & flavors within a single project; it should support this use case. Conceptually it seems the most sensible way to set up Gradle-based protobuf code generation when Android is involved.

Some problems I encountered:
- Single Gradle protobuf block not allowing me to specify different codegen options for builtins & plugins for the various multiplatform targets, in particular `option("lite")`.
- Plugin generates code only for Android build configurations "debug" and "release". No code is generated for the "jvm" multiplatform target (the server protobuf sources).
- Plugin by default compiles protobuf source from "src/main/proto" rather than "src/commonMain/proto" or any of the other actual multiplatform sourcesets.

Start by tracing the protobuf Gradle plugin's handling of builtins and plugins across Kotlin multiplatform targets, especially Android debug/release and the jvm target. Compare how it selects protobuf sources from src/main/proto versus multiplatform source sets. Done means one project can generate the requested Android lite and server Java sources with target-specific options.

Written by the indexing model from the issue text.