Support loading plugins from multiple directories

At Stripe, we have a use case that we would like Pinot to load plugins from it's install directory, but also additional directories where we have our plugin jars. I've created a pull request adding this support here: https://github.com/apache/pinot/pull/7871

Start by reviewing the pull request linked in the issue, https://github.com/apache/pinot/pull/7871, to identify the affected plugin-loading entry points and any tests. Done means Pinot can load plugins from its install directory and from additional directories containing plugin JARs.

Written by the indexing model from the issue text.