Allow configuring the final R8 configuration file with R8Spec.configurationFile

[Goooler]

Closes #2128.

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• CHANGELOG's "Unreleased" section has been updated, if applicable.

[Copilot]

Pull request overview

This PR introduces a new R8Spec.configurationFile property to control where R8 writes the final/collective ProGuard configuration (via --pg-conf-output), and updates the R8 rules generation so relative report paths can be resolved from the chosen configuration file directory.

Changes:

• Add R8Spec.configurationFile (defaulting to build/shadowJar/r8/configuration.txt) and wire it into R8 execution.
• Prepend a -basedirectory rule derived from configurationFile’s directory to influence how R8 resolves relative report paths.
• Add/extend unit + functional tests, and update docs/changelog/API surface.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/test/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/MinimizeSpecsTest.kt	Adds coverage for the new default configurationFile convention.
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/tasks/R8Spec.kt	Introduces the new configurationFile property on the public R8 DSL/API.
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/R8Minimizer.kt	Passes --pg-conf-output and injects -basedirectory into generated rules.
src/main/kotlin/com/github/jengelman/gradle/plugins/shadow/internal/DefaultR8Spec.kt	Sets the default convention for configurationFile under build/shadowJar/r8/.
src/functionalTest/kotlin/com/github/jengelman/gradle/plugins/shadow/MinimizeTest.kt	Adds functional tests validating configuration output content and report path behavior.
docs/configuration/minimizing/README.md	Documents configurationFile and how R8 report paths resolve relative to it.
docs/changes/README.md	Adds an Unreleased changelog entry for R8Spec.configurationFile.
api/shadow.api	Updates the public API signature to include getConfigurationFile().

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