Refactor bin/solr to allow external override of Jetty modules

The bin/solr script currently does not allow for externally overriding the modules passed to Jetty on startup.

This PR adds the ability to override the Jetty modules on startup by setting JETTY_MODULES as an environment variable; when passed, bin/solr will pass through (and not clobber) the string verbatim into SOLR_JETTY_CONFIG. For example, you can now run:

JETTY_MODULES=--module=foo bin/solr start

We've added some custom Jetty modules that can be optionally enabled; this change allows us to keep our logic (regarding which modules to use) in a separate script, rather than maintaining a forked bin/solr.