[Feature Request] Support for mapstruct-spi-protobuf

Protobuf is widely used in the industry and often we need to convert proto generated classes to/from POJOs (or models generated by Lombok/AutoValue/Immutables/YouNameIt).

While it would be awesome for MapStruct to provide out of the box integration with Protobuf like Lombok. Before that it seems that the next best choice is to use this SPI https://github.com/entur/mapstruct-spi-protobuf.

This SPI solves the compile time problem where MapStruct thinks there are unmapped target properties.

However the Idea plugin is not recognizing the SPI and still shows a warning. It would be super nice the plugin would match the complier's behaviour in that.

Start by examining how the IntelliJ plugin detects unmapped target properties and compare that behavior with the compiler when using the mapstruct-spi-protobuf integration. Done means the plugin no longer reports a warning for protobuf-generated classes when the SPI makes the mapping valid.

Written by the indexing model from the issue text.