The easiest way would be to build a single "fat jar" that contains everything that you need using SBT and load it into databricks.
Assuming you have SBT installed locally, roughly the steps are
1. Create an sbt project with ScalaPB using:
sbt new scalapb/scalapb-template.g8
2. Add your protobufs into src/main/protobuf
3. Add scalpb-json4s to the project as a library dependency.
4. Add sbt-assembly to your project. See steps here
You may also need to shade Jackson in case you are getting some MethodNotFound errors.
5. Run "sbt assembly" to generate the fat jar.
6. Upload the generated JAR into databricks.
Hope it helps,