Gradle should allow pom.xml customization for Gradle Plugin Portal artifacts

### Expected Behavior

There should be a way to ensure `pom.xml` files are identical at Gradle Plugin Portal and at Sonatype OSSRH.

### Current Behavior

Gradle deploys a different pom to the portal, so the users of Gradle Dependency Verification identify an unexpected difference: https://github.com/burrunan/gradle-s3-build-cache/issues/8

### Context

I deploy the plugin to both OSSRH and Portal (mainly for testing and signing reasons), and OSSRH mandates certain elements in the pom (license, etc).

Start by comparing the pom.xml produced for the Gradle Plugin Portal with the one published to Sonatype OSSRH, focusing on how Gradle deployment currently handles customization. The work is done when a plugin can produce identical POM files at both destinations, including required elements such as the license.

Written by the indexing model from the issue text.