New Maven provider: Clojars

[Clojure](https://clojure.org/) is a popular functional Lisp dialect that's hosted on the JVM, and leverages Maven artifact repositories for package management and deployment. While some Clojure projects are deployed to Maven Central (notably [the core language itself](https://mvnrepository.com/artifact/org.clojure/clojure)), [Clojars](https://clojars.org/) has become the de-facto standard artifact repository for most community-developed Clojure open source projects. At the time of writing Clojars hosts approximately 32,000 unique projects (as defined by groupId+artifactId), totaling almost 300,000 distinct artifacts (i.e. groupId+artifactId+version).

Clojars is a well-managed, secure, and fully Maven-compatible artifact repository, and so it is my belief that it [meets ClearlyDefined's criteria for consideration as a new provider](https://docs.clearlydefined.io/docs/get-involved/adding-sources#adding-a-new-harvest-source).

More generally, there are other widely used Maven artifact repositories that may also be of interest to other JVM developers as ClearlyDefined providers. [mvnrepository.com's repository index](https://mvnrepository.com/repos) is a good place to get a better sense of the broader Maven ecosystem and how it is used by JVM developers (especially those who aren't restricted to Java-the-language and/or Maven Central).