Provide an alternative without redirects

[ov7a]

As a follow-up to

• Add option for Gradle Plugin Repository to NOT redirect to JCenter as they regularly rewrite POMs and break checksums gradle#15406

In some contexts, it makes more sense to use the Plugin portal only for the artifacts published there and have no redirect to the Maven Central.

We should consider providing an alternative entry point for these scenarios.
Note that the current behavior will be preserved by default for compatibility. One important aspect here is plugin dependencies that are usually on Maven Central: without redirects, they won't be found. So Maven Central repository should be declared along with the Plugin Portal Repository to use them.

We might consider this option in future versions of Gradle.

[justintuchek]

We have discovered a number of areas where the Gradle Plugin repository is referenced in areas where it probably shouldn't be (ex: library repositories, buildscript repositories) and to my surprise working successfully because of these redirects

This ended up being an unexpected contributor to Maven Central rate-limiting because of redirects being followed either from builds themselves or internal Artifactory/Sonatype instances following these to a 429 upstream - references on these changes

• https://www.sonatype.com/blog/maven-central-and-the-tragedy-of-the-commons
• https://www.sonatype.com/blog/beyond-ips-addressing-organizational-overconsumption-in-maven-central

Here's an example that does work that probably shouldn't https://plugins.gradle.org/m2/com/graphql-java/graphql-java/22.4/graphql-java-22.4.jar

Having an option without redirects would allow us to update our internal mirrors so this can be restricted to its intended purpose and help mitigate traffic dumping back to Maven Central. We can fix these unusual declarations as well but this would help prevent scenarios like this from happening again in the future