Gradle: Allow setting custom sourceset

[stashymane]

Environment:

• Jib version: 3.4.3
• Build tool: Gradle 8.7

Description of the issue:
Currently, the main sourceset is hardcoded into the gradle plugin. With the default Java plugin this works well, however running anything else makes using Jib difficult.
My use-case is Kotlin Multiplatform - it does generate a JVM target, however it does not create a main sourceset. This results in Jib throwing an exception:

org.gradle.api.ProjectConfigurationException: A problem occurred configuring project ':server'.
Caused by: org.gradle.api.UnknownDomainObjectException: SourceSet with name 'main' not found.
	at com.google.cloud.tools.jib.gradle.JibPlugin.lambda$apply$9(JibPlugin.java:189)

Having to manually set up the main sourceset as a "mirror" is error-prone and cumbersome. A very simple choice of sourceset in the Jib configuration would solve most issues.

This exact issue was already mentioned in #1778, and even though the issue was closed, only the configuration was allowed to be configurable. The sourceset still isn't.

[yluom]

Out of curiosity, @stashymane could you explain how you bypassed this problem ?

[stashymane]

@yluom specifically for Kotlin Multiplatform, turns out you can run withJava() on the JVM configuration block. I avoided this earlier due to some issues being associated with it, but I'm not sure what that was anymore.
In case of any other configuration, no clue.

[chris-hatton]

With Kotlin 2.1.20 now incompatible with the application plugin and the withJava() API now deprecated, it could become relevant to consider adding configurations to Jib, to support KMP/JVM users who have an e.g. jvmMain source-set instead of plain main.

[osipxd]

It seems like the only way to bypass this problem is to create a new JVM-only project with JIB applied and add the KMP project as a dependency to it.

[nathanfallet]

Maybe there is a way to configure this, or at least check for multiple targets? (Like you check for "main" ?: "jvmMain" ?: "commonMain", you get the idea)
EDIT: I inspected the code, I'm making a PR that allows to configure it via the configuration extension of the Gradle plugin