[Feature Request] Support for mapstruct-spi-protobuf

[91wzhang]

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.