[JavaSpring, kotlin-spring] attach sources when maven and interfaceOnly=true is used

[zaenk]

When using interfaceOnly, the configuration effectively generates only API interfaces and DTOs. The generated sources can be packaged up and published to a maven repository to be used as an "SDK" for both client and server development.

For these generated libraries, when the developer navigates to definitions, the decomplied code is not easily readable, especially true in case of decompiled kotlin files.

Closes #15659

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[wing328]

circleci failure not related to this PR

[wing328]

thanks for the PR

let's give it a try

[wing328]

FYI. Just merged #20647 to improve the mustache template to avoid blank lines.

[zaenk]

this was super fast! thank you! 🙏