Support Spring shading and relocation

[vlsi]

What feature do you want to see added?

It would be nice if there was a possibility to configure Spring shading and relocation.

There's META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports which can currently be handled with ServiceFileTransformer, however, it would be nice if out-of-the-box SpringTransfomer would handle it as well.

There's META-INF/spring-configuration-metadata.json which contains helper information for IDE, and we can probably ignore it for now, however, transforming and merging that file would be a plus.

Spring has several files which should be modified for proper relocation:

        "META-INF/spring-autoconfigure-metadata.properties",
        "META-INF/spring.factories",
        "META-INF/spring.handlers",
        "META-INF/spring.schemas",
        "META-INF/spring.tooling",

The files are in Properties format, however:

1. schemas and tooling have path for the values
2. The rest have class for the values
3. factories has class for the values

Sometimes, the values are comma-separated, so merging should use comma-separated value.

I've implemented SpringFileTransformer which works for me, however, it would be nice if there was out-of-the-box one.
Even though shading Spring is awkward, there are cases when it is needed. See also:

• My implementation (Apache 2.0 code ahead): https://github.com/Netcracker/qubership-profiler-agent/pull/192/files#diff-e17241eb65fd812b70ff17dbad8b4e643a464aa329220590758e689e12e2abb9R13
• Document using the Shadow plugin as an alternative to Boot's uber jars when using Gradle spring-projects/spring-boot#1828.

Are you interested in contributing this feature?

I might contribute, however, I am not comfortable with writing Groovy code.

[Goooler]

PRs are welcome! Shadow has been rewritten for Kotlin!

[Goooler]

Seems we can handle this by creating a composite ResourceTransformer which delegates to PropertiesFileTransformer.